-Choose ONE of these re-emerging diseases…Measles, Mumps or Cholera-Locate an article in a newspaper, in a lay magazine, or on anorganizational website that discusses an emerging or re-emerginginfectious disease that is currently affecting your community. Reflecton the article you selected and think about how the emerging orre-emerging disease might affect nursing practice-Briefly summarize the article you selected and provide the reference. Then, address the following: What implications does the disease have for nursing practice? How does the emergence of this disease affect your personal practice? Why do you think that this disease is emerging/re-emerging? What is the nurse’s role in preventing and managing the impact ofinfectious diseases both from the patient and nurse perspective? Support your response with references from the professional nursing literature. 3-paragraph (at least 350 words) Be sure to use evidence from the readings and include in-text citations. A reference list is required. Use the most current evidence (usually ≤ 5 years old) National Institutes of Health (US);Biological Sciences Curriculum Study. NIH Curriculum Supplement Series[Internet]. Bethesda (MD): National Institutes of Health (US); 2007.Understanding Emerging and Re-emerging Infectious Diseases. Availablefrom: https://www.ncbi.nlm.nih.gov/books/NBK20370/NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.National Institutes of Health (US); Biological Sciences CurriculumStudy. NIH Curriculum Supplement Series [Internet]. Bethesda (MD):National Institutes of Health (US); 2007. Understanding Emerging and Re-emerging Infectious DiseasesThe term “disease” refers to conditions that impair normal tissuefunction. For example, cystic fibrosis, atherosclerosis, and measles areall considered diseases. However, there are fundamentally differentcauses for each of these diseases. Cystic fibrosis (CF) is due to aspecific genotype that results in impaired transport of chloride ionsacross cell membranes, leading to the production of abnormally thickmucus. Thus, CF is most accurately called a genetic or metabolic disease. Atherosclerosis, which can lead to heart attacks and strokes, may be considered a disease of aging,because it typically becomes a problem later in life after plaques ofcholesterol have built up and partially blocked arteries. In contrast,measles is an infectious disease because it occurs when an individual contracts an outside agent, the measles virus. An infectious disease is a disease that is caused by the invasion of a host by agents whose activities harm the host’s tissues (that is, they cause disease) and can be transmitted to other individuals (that is, they are infectious). Nature of Infectious DiseasesMicroorganisms that are capable of causing disease are called pathogens.Although microorganisms that cause disease often receive the mostattention, it is important to note that most microorganisms do notcause disease. In fact, many probably provide some protection againstharmful microorganisms because they effectively compete with the harmfulorganisms for resources, preventing them from growing.A true pathogen is an infectious agent that causes disease invirtually any susceptible host. Opportunistic pathogens are potentiallyinfectious agents that rarely cause disease in individuals with healthyimmune systems. Diseases caused by opportunistic pathogens typically arefound among groups such as the elderly (whose immune systems arefailing), cancer patients receiving chemotherapy (which adverselyaffects the immune system), or people who have AIDS or are HIV-positive.An important clue to understanding the effect of HIV on the immunesystem was the observation of a rare type of pneumonia among young mencaused by Pneumocystis carinii, an organism that causes disease only among the immunosuppressed.The terms “infection” and “disease” are not synonymous. An infection results when a pathogen invades and begins growing within a host. Diseaseresults only if and when, as a consequence of the invasion and growthof a pathogen, tissue function is impaired. Our bodies have defensemechanisms to prevent infection and, should those mechanisms fail, toprevent disease after infection occurs. Some infectious agents areeasily transmitted (that is, they are very contagious), but they are notvery likely to cause disease (that is, they are not very virulent). Thepolio virus is an example: It probably infects most people who contactit, but only about 5 to 10 percent of those infected actually developclinical disease. Other infectious agents are very virulent, but notterribly contagious. The terror surrounding Ebola hemorrhagic fever isbased on the virulence of the virus (50 to 90 percent fatality rateamong those infected); however, the virus itself is not transmittedeasily by casual contact. The most worrisome infectious agents are thosethat are both very contagious and very virulent.In order to cause disease, pathogens must be able to enter the hostbody, adhere to specific host cells, invade and colonize host tissues,and inflict damage on those tissues. Entrance to the host typicallyoccurs through natural orifices such as the mouth, eyes, or genitalopenings, or through wounds that breach the skin barrier to pathogens.Although some pathogens can grow at the initial entry site, most mustinvade areas of the body where they are not typically found. They dothis by attaching to specific host cells. Some pathogens then multiplybetween host cells or within body fluids, while others such as virusesand some bacterial species enter the host cells and grow there. Althoughthe growth of pathogens may be enough to cause tissue damage in somecases, damage is usually due to the production of toxins or destructiveenzymes by the pathogen. For example, Corynebacterium diphtheriae,the bacteria that causes diphtheria, grows only on nasal and throatsurfaces. However, the toxin it produces is distributed to other tissuesby the circulatory system, damaging heart, liver, and nerve tissues. Streptococcus pyogenes,the infectious agent associated with several diseases includingstrepthroat and “flesh-eating disease,” produces several enzymes thatbreak down barriers between epithelial cells and remove fibrin clots,helping the bacteria invade tissues. Microbes That Cause Infectious DiseasesThere are five major types of infectious agents: bacteria, viruses,fungi, protozoa, and helminths. In addition, a new class of infectiousagents, the prions, has recently been recognized. A brief review of thegeneral characteristics of each of these agents and examples of somediseases they cause follows. BacteriaBacteria are unicellular prokaryotic organisms; that is, they have noorganized internal membranous structures such as nuclei, mitochondria,or lysosomes. Their genomes are circular, double-stranded DNA that isassociated with much less protein than eukaryotic genomes. Most bacteriareproduce by growing and dividing into two cells in a process known asbinary fission. Despite these commonalities that group them together inthe Kingdom Monera, there is a wide range of diversity among thebacteria.There are a variety of morphologies among bacteria, but three of themost common are bacillus (rod-shaped), coccus (spherical), or spirillum(helical rods). The energy sources for bacteria also vary. Some bacteriaare photosynthetic and obtain their energy directly from the sun.Others oxidize inorganic compounds to supply their energy needs. Stillother bacteria generate energy by breaking down organic compounds suchas amino acids and sugars in a respiratory process. Some bacteriarequire oxygen (aerobes), while others are unable to tolerate it(anaerobes). Some bacteria can grow either with or without oxygen(facultative anaerobes).Bacteria are frequently divided into two broad classes based on theircell wall structures, which influences their Gram stain reaction.Gram-negative bacteria appear pink after the staining procedure.Familiar pathogenic gram-negative organisms are Salmonella typhi, which causes typhoid fever, and Yersiniapestis,which causes plague. Gram-positive bacteria appear purple after theGram stain procedure. Examples of pathogenic gram-positive bacteria are Staphylococcus aureus, which causes skin, respiratory, and wound infections, and Clostridium tetani, which produces a toxin that can be lethal for humans. VirusesMicrobiologists have found viruses that infect all organisms, fromplants and animals to fungi and bacteria. Viruses, however, are notorganisms themselves because, apart from a host cell, they have nometabolism and cannot reproduce. A virus particle is composed of a viralgenome of nucleic acid that is surrounded by a protein coat called acapsid. In addition, many viruses that infect animals are surrounded byan outer lipid envelope, which they acquire from the host cell membraneas they leave the cell. Unlike organisms, in which the genetic materialis always double-stranded DNA, viral genomes may be double- orsingle-stranded DNA (a DNA virus), or double- or single-stranded RNA (anRNA virus).In the general process of infection and replication by a DNA virus, aviral particle first attaches to a specific host cell via proteinreceptors on its outer envelope, or capsid. The viral genome is theninserted into the host cell, where it uses host cell enzymes toreplicate its DNA, transcribe the DNA to make messenger RNA, andtranslate the messenger RNA into viral proteins. The replicated DNA andviral proteins are then assembled into complete viral particles, and thenew viruses are released from the host cell. In some cases,virus-derived enzymes destroy the host cell membranes, killing the celland releasing the new virus particles. In other cases, new virusparticles exit the cell by a budding process, weakening but notdestroying the cell.In the case of some RNA viruses, the genetic material can be useddirectly as messenger RNA to produce viral proteins, including a specialviral RNA polymerase that copies the RNA template to produce thegenetic material for new viral particles. Other RNA viruses, calledretroviruses, use a unique enzyme called reverse transcriptase to copythe RNA genome into DNA. This DNA then integrates itself into the hostcell genome. These viruses frequently exhibit long latent periods inwhich their genomes are faithfully copied and distributed to progenycells each time the cell divides. The human immunodeficiency virus(HIV), which causes AIDS, is a familiar example of a retrovirus.Just like other infectious agents, viruses cause disease bydisrupting normal cell function. They do this in a variety of ways. Someviruses make repressor proteins that stop the synthesis of the hostcell’s proteins, RNA, and DNA. Viral activity may weaken cell membranesand lysosomal membranes, leading to cell autolysis. Some viral proteinsare toxic to cells, and the body’s immune defenses also may killvirus-infected cells.Viruses are classified using a variety of criteria, including shape,size, and type of genome. Among the DNA viruses are the herpes virusesthat cause chicken pox, cold sores, and painful genital lesions, and thepoxvirus that causes smallpox. Significant RNA viruses that cause humandisease include rhinoviruses that cause most common colds; myxovirusesand paramyxoviruses that cause influenza, measles, and mumps;rotaviruses that cause gastroenteritis; and the retroviruses that causeAIDS and several types of cancer. FungiFungi are eukaryotic, heterotrophic organisms that have rigidcellulose- or chitin-based cell walls and reproduce primarily by formingspores. Most fungi are multicellular, although some, such as yeasts,are unicellular. Together with bacteria, fungi fulfill the indispensablerole of decomposers in the environment. Many fungi also infect plantsand animals. Examples of diseases caused by fungi are ringworm andhistoplasmosis (a mild to severe lung infection transmitted by bat orbird droppings). Yeasts of the Candida genus are opportunisticpathogens that may cause diseases such as vaginal yeast infections andthrush (a throat infection) among people who are immunocompromised orundergoing antibiotic therapy. Antibiotics reduce the bacterialpopulation normally present in the throat and vagina, allowing the yeastto grow unchecked. ProtozoaProtozoa are unicellular, heterotrophic eukaryotes that include thefamiliar amoeba and paramecium. Because protozoa do not have cell walls,they are capable of a variety of rapid and flexible movements. Protozoacan be acquired through contaminated food or water or by the bite of aninfected arthropod such as a mosquito. Diarrheal disease in the UnitedStates can be caused by two common protozoan parasites, Giardia lamblia and Cryptosporidium parvum.Malaria, a tropical illness that causes 300 million to 500 millioncases of disease annually, is caused by several species of the protozoanPlasmodium. HelminthsHelminths are simple, invertebrate animals, some of which areinfectious parasites. They are multicellular and have differentiatedtissues. Because they are animals, their physiology is similar in someways to ours. This makes parasitic helminth infections difficult totreat because drugs that kill helminths are frequently very toxic tohuman cells.Many helminths have complex reproductive cycles that include multiple stages, many or all of which require a host. Schistosoma, a flatworm, causes the mild disease swimmer’s itch in the United States; another species of Schistosomacauses the much more serious disease schistosomiasis, which is endemicin Africa and Latin America. Schistosome eggs hatch in freshwater, andthe resulting larvae infect snails. When the snails shed these larvae,the larvae attach to and penetrate human skin. They feed, grow, and matein the human bloodstream; the damage to human tissues caused by theaccumulating schistosome eggs with their sharp spines results in diseasesymptoms including diarrhea and abdominal pain. Liver and spleeninvolvement are common. Another disease due to a helminth istrichinosis, caused by the roundworm Trichinella spiralis. Thisinfectious agent is typically ingested in improperly cooked pork frominfected pigs. Early disease symptoms include vomiting, diarrhea, andfever; later symptoms include intense muscle pain because the larvaegrow and mature in those tissues. Fatal cases often show congestiveheart failure and respiratory paralysis. PrionsDuring the past two decades, evidence has linked some degenerativedisorders of the central nervous system to infectious particles thatconsist only of protein. These “proteinaceous infectious particles” havebeen named prions (preeons). The known prion diseases includeCreutzfeldt-Jakob disease (in humans), scrapie (in sheep), and bovinespongiform encephalopathy (“mad cow disease” in cattle); all known priondiseases frequently result in brain tissue that is riddled with holes.While some prion diseases are inherited, others are apparently due toinfection by eating infected tissue or inadvertently through medicalprocedures such as tissue transplants. Occurrence of Infectious Diseases Epidemiology is the study of the occurrence of disease inpopulations. Epidemiologists are concerned not only with infectiousdiseases, but also with noninfectious diseases such as cancer andatherosclerosis, and with environmental diseases such as lead poisoning.These professionals work to prevent or minimize the impact of diseasesin the population. Their work may include such activities as identifyingunusually high incidences of a particular disease, determining theeffectiveness of a vaccine, and cal collating the cost effectiveness ofvarious means of controlling disease transmission. Occasionally,epidemiologists act as “detectives” who track down the cause of a “new”disease, determine its reservoir and mode of transmission, and helporganize various health care workers to bring the disease under control. Disease reservoirsThe reservoir for a disease is the site where the infectious agentsurvives. For example, humans are the reservoir for the measles virusbecause it does not infect other organisms.Animals often serve as reservoirs for diseases that infect humans. The major reservoir for Yersinia pestis,the bacteria that causes plague, is wild rodents. There are alsononliving reservoirs. Soil is the reservoir for many pathogenic fungi aswell as some pathogenic bacteria such as Clostridium tetani, which causes tetanus. Modes of transmissionInfectious agents may be transmitted through either direct orindirect contact. Direct contact occurs when an individual is infectedby contact with the reservoir, for example, by touching an infectedperson, ingesting infected meat, or being bitten by an infected animalor insect. Transmission by direct contact also includes inhaling theinfectious agent in droplets emitted by sneezing or coughing andcontracting the infectious agent through intimate sexual contact. Somediseases that are transmitted primarily by direct contact with thereservoir include ringworm, AIDS, trichinosis, influenza, rabies, andmalaria.Indirect contact occurs when a pathogen can withstand the environmentoutside its host for a long period of time before infecting anotherindividual. Inanimate objects that are contaminated by direct contactwith the reservoir (for example, a tissue used to wipe the nose of anindividual who has a cold or a toy that has been handled by a sickchild) may be the indirect contact for a susceptible individual.Ingesting food and beverages contaminated by contact with a diseasereservoir is another example of disease transmission by indirectcontact. The fecal-oral route of transmission, in whichsewage-contaminated water is used for drinking, washing, or preparingfoods, is a significant form of indirect transmission, especially forgastrointestinal diseases such as cholera, rotavirus infection,cryptosporidiosis, and giardiasis.These modes of transmission are all examples of horizontaltransmission because the infectious agent is passed from person toperson in a group. Some diseases also are transmitted vertically; thatis, they are transmitted from parent to child during the processes ofreproduction (through sperm or egg cells), fetal development, or birth.Diseases in which vertical transmission occurs include AIDS and herpesencephalitis (which occurs when an infant contracts the herpes simplextype II virus during vaginal birth). Role of Research in PreventionInfectious diseases can be prevented at a variety of points, depending on the infectious cycle for the particular disease (Figure 4).Basic research, such as that sponsored by NIH, reveals the specificinfectious cycle and details regarding the activities of the pathogenthat cause disease (for example, the particular cells, if any, that areattacked, and the toxins produced by the pathogen that damage hosttissues).Understanding the infectious cycle is critical in order to identify accessible targets for control strategies (Figure 4).For example, direct person-to-person transmission may be inhibited byproper hygiene and sanitary conditions as well as education.Vector-borne diseases may be prevented by control measures that eitherkill the vector or prevent its contact with humans. Infection by apathogen or development of a pathogen within a host may be prevented byvaccination. Finally, drugs may be used to prevent infection or suppressthe disease process.In some cases, the tools, including drugs, vaccines and vectorcontrol methods, are already available to deal with these diseases. Forother diseases, the methods for control are inadequate, undeveloped, ornonexistent. Scientists are trying to develop the new tools needed tobanish these scourges of mankind. This requires basic research into thelife processes of the pathogen and its interaction with the host inorder to identify points within the life cycle where the pathogen isvulnerable to intervention, translational research to develop new tools(such as vaccines or antimicrobial drugs), and clinical research to testthe safety and efficacy of these new tools. Host Defenses Against Infectious DiseasesThe human body has several general mechanisms for preventinginfectious diseases. Some of these mechanisms are referred to asnonspecific defenses because they operate against a wide range ofpathogens. Other mechanisms are referred to as specific defenses becausethey target particular pathogens and pathogen-infected cells. Nonspecific mechanismsNonspecific mechanisms are the body’s primary defense againstdisease. These mechanisms include anatomical barriers to invadingpathogens, physiological deterrents to pathogens, and the presence ofnormal flora. An example of an anatomical barrier is the nasal openingto the respiratory system. This natural opening is a long, convolutedpassage covered by mucous membranes that trap airborne particles andprevent most of them from reaching the lungs. Other anatomical barriersare the skull and vertebral column, which protect the central nervoussystem— few pathogens are able to penetrate bone. The skin also is amajor anatomical barrier to microorganisms. The surface layer of dead,hardened cells is relatively dry, and skin secretions make the surfacesomewhat acidic. When sweat evaporates, salt is left behind on the skin.All of these conditions (low moisture, low pH, and high salinity)prevent most microorganisms from growing and multiplying on the skin.The major medical challenge in treating burn patients is preventing andtreating infections that result because of the absence of skin thatordinarily would prevent invasion of microorganisms.Natural openings also are protected by a variety of physiologicaldeterrents. For example, tears continually flush debris from the eyes.Vaginal secretions are acidic, a hostile environment that discouragesthe growth of many pathogens. The eye, mouth, and nasal openings areprotected by tears, saliva, or nasal secretions that contain lysozyme,an enzyme that breaks down bacterial cell walls. Blood, sweat, and sometissue fluids contain lysozyme as well.In addition to lysozyme, the blood has many elements that defend thebody from disease-causing organisms. The white blood cells includeseveral types of phagocytic cells that detect, track, engulf, and killinvading bacteria and viruses, as well as infected host cells and otherdebris. These phagocytic cells are part of the nonspecific immune system. Blood plasma also includes clotting factors that initiate a clot atthe injury site, preventing pathogens from invading the body further.Finally, the complement proteins in the blood participate in a cascadeof molecular events that result in inflammation, the release ofmolecules that stimulate phagocytic cells, and the formation of acomplex of proteins that binds to the surface of bacterial or infectedhost cells and lyses those cells.The inflammatory response is another nonspecific defense mechanismthat helps prevent infectious agents from spreading in the body.Inflammation involves swelling, reddening, elevated temperature, andpain. Unfortunately, inflammation itself frequently causes tissue damageand, in severe cases, even death.Finally, the protective role of the “normal flora” of microorganismspresent on and in the body should not be overlooked. These organismssurvive and grow on the skin and in the mouth, gastrointestinal tract,and other areas of the body, but do not cause disease because theirgrowth is kept under control by the host’s defense mechanisms and by thepresence of other microorganisms. These organisms protect the host bysuccessfully competing with disease-causing organisms, preventing thelatter from invading host tissues. When the growth of the normal florais suppressed (for example, due to antibiotic treatment), other“opportunistic” agents that normally do not grow in or on the body maybe able to infect and cause disease. Specific mechanisms of host resistanceWhen these nonspecific mechanisms fail, the body initiates a second,specific line of defense. This specific immune response enables the bodyto target particular pathogens and pathogen-infected cells fordestruction. It depends on specialized white blood cells calledlymphocytes and includes T-cells (produced from lymphocytes that maturedin the thymus gland) and B-cells (produced from lymphocytes thatmatured in the bone marrow).The two complementary components of the specific immune response arethe cell-mediated response and the antibody-mediated response (Figure 5).The cell-mediated response involves T-cells and is responsible fordirectly destroying body cells that are infected with a virus or havebecome cancerous, or for activating other immune cells to be moreefficient microbe killers. The antibody-mediated response involves bothT-cells and B-cells and is critical for the destruction of invadingpathogens as well as the elimination of toxins.Both the cell-mediated and antibody-mediated responses are initiatedafter a particular type of phagocytic cell, a macrophage, engulfs apathogen. Macrophages digest the pathogen and then display antigens fromthe pathogen on their surface. Antigens are specific molecules, such asthe proteins on the surface of pathogens, that elicit an immuneresponse. This display helps the macrophages stimulate specific helperT-cells to release signal molecules called lymphokines. The lymphokines,in turn, stimulate the cell-mediated and antibody-mediated responses.The cell-mediated response occurs when the lymphokines released fromthe helper T-cells stimulate other cell types to participate in theimmune response. Lymphokine-stimulated killer T-cells attach to thepathogen-infected cells and destroy them, whereas lymphokine-activatedphagocytic cells produce more toxic molecules that can kill the pathogendirectly.The antibody-mediated response occurs when the lymphokines activatespecific B-cells to produce antibodies (proteins that specificallyrecognize and bind to antigens). These antibodies attach to antigens onthe surface of the pathogens and signal attack by phagocytic cells andcomplement system. Other B-cells go on to become memory B-cells, whichrespond quickly by producing more antibodies upon subsequent infection. ImmunityWhen a host encounters an antigen that triggers a specific immuneresponse for the second or later time, the memory lymphocytes recognizeit and quickly begin growing and dividing, as well as producing highlevels of lymphokines and antibodies. Because memory cells are present,this response happens much more quickly than in the initial encounterwith the antigen. This rapid response explains why hosts are immune todeveloping many diseases a second time: The immune response occurs soquickly in a second encounter with the pathogen that the pathogen doesnot have enough time to reproduce to levels that result in diseasebefore the host’s body has destroyed it. The memory response alsoexplains the effectiveness of vaccination for preventing even the firstoccurrence of many diseases. VaccinationA vaccine is either a killed or weakened (attenuated) strain of aparticular pathogen, or a solution containing critical antigens from thepathogen. The body’s immune system will respond to these vaccines as ifthey contain the actual pathogen, even though the vaccine is notcapable of causing the disease. As a result of the specific immuneresponse, memory lymphocytes will be present that respond rapidly whenthe actual pathogen is encountered. The resulting rapid activation ofimmune cells prevents disease.Currently new types of vaccines, the DNA vaccines, are in early stagetrials. These vaccines contain genes that encode proteins frompathogens. When these genes are inserted into host cells and areexpressed in the form of pathogen proteins, an immune reaction mayresult.The ultimate effectiveness of vaccination—eradication of theinfectious agent—has been achieved only for smallpox. The World HealthOrganization has identified the polio and measles viruses among the nexttargets for global eradication.For a variety of reasons, many diseases are not easily prevented byvaccination. Antibody response is generally the simplest to induce byvaccination, but some pathogens have ways to evade the immune response.Intracellular pathogens (such as viruses and some bacterial andprotozoan pathogens) are not directly affected by antibodies becauseantibodies cannot pass inside cells. Moreover, during the diseaseprocess, some pathogens acquire an external coat composed ofhost-derived material while others disguise themselves by makingmolecules that resemble host molecules. Thus, the host’s immune systemdoes not identify them as foreign invaders. Still other pathogens mutatequickly, producing variants of their antigens that are not recognizedby the host’s immune system, even though the host survived a previousencounter with that pathogen. Cold and influenza viruses are examples ofrapidly mutating pathogens. Scientists are working to improve vaccinesagainst these pathogens Public Health Measures to Prevent Infectious DiseasesDeveloped countries have regulations that help protect the generalpublic from infectious diseases. Public health measures typicallyinvolve eliminating the pathogen from its reservoir or from its route oftransmission. Those measures include ensuring a safe water supply,effectively managing sewage treatment and disposal, and initiating foodsafety, animal control, and vaccination programs. Safe waterMany pathogens that cause gastrointestinal diseases (for example,those that cause cholera and typhoid fever) are transmitted via water.Travelers to developing countries are frequently advised to be immunizedagainst these diseases. This is generally unnecessary in the UnitedStates and other developed countries because the water used for washing,drinking, and preparing food is purified before it goes into homes.Purification methods include settling, filtration, and chlorination. Thewater for homes that use well water or springs is usually safe ifguidelines about distance from sewage disposal facilities are followed;however, this water should be checked periodically. When breakdowns in apurification sys tem occur, or when a system is overwhelmed (forexample, due to unusual flooding), drinking water may not be safe andshould be boiled or treated with chlorine before it is ingested.Because gastrointestinal pathogens typically leave the body in thefeces, public water must be guarded against contamination from sewage.Municipal water is usually tested for the presence of coliform organisms(nonpathogenic microorganisms that are part of the normal flora of thegastrointestinal tract) as indicators of sewage contamination. Thisprocedure is necessary because when the water contains pathogens and ispotentially dangerous, the pathogenic organisms are usually present insuch small numbers that they are hard to detect. Sewage treatment and disposalSewage includes wash water, water from toilets, and storm run-off.These fluids may carry the pathogens for many waterborne diseases,including giardiasis and hepatitis A; therefore, to ensure public safetythe U.S. government (and the governments of other developed countries)requires that sewage be treated to eliminate pathogens. The minimalacceptable level of treatment involves collection and sedimentation ofsewage waters, separating solid matter (sludge) from the liquid(effluent) portion of sewage. The effluent is chlorinated to killpathogens before it is released to rivers or lakes. The sludge is burnedor dumped.More advanced methods of treatment use a secondary treatmentfollowing this primary treatment. The effluent is transferred to tankscontaining a population of microorganisms that decompose more than 90percent of the organic wastes and eliminate pathogens by competition(this is another example of the important role of microorganisms in preventingdisease). The resulting effluent is chlorinated before it is releasedto the environment. Some sewage treatment plants include a tertiarytreatment that involves additional chemicals that also eliminatepathogens. Food safety programsThe United States has many standards, inspection plans, andregulations about food preparation, handling, and distribution.Meat-packing facilities are inspected regularly to detect and eliminatediseased animals, ensure that standards for processes such as meatcutting and refrigeration are observed, and detect residues frompesticides and antibiotics as well as contamination by bacteria andother parasites. Restaurants and supermarkets are similarly inspected.Milk is pasteurized and dated for sale and is analyzed periodically forcontamination. Industry standards for canning and preserving foods aremaintained through periodic quality control checks and, if contaminationis found in representatives of any batches, public health officialsrecall the entire batch and alert the public through the media. Animal control programsAnimals are carriers of many diseases that also affect humans.Inspecting domestic herd animals for tuberculosis (due to the bacterium Mycobacterium bovis)and brucellosis (a disease that causes spontaneous abortion in domesticherd animals and abscesses of the liver, spleen, bone marrow, and lymphnodes in humans) has helped eliminate the threat of passing thepathogens for those diseases to humans in contaminated milk and meat.Before their pets can be licensed, dog owners must show proof of rabiesvaccination. Because most cases of rabies among people in the UnitedStates are due to bites from wild and stray animals, health officialsare mandated to impound and destroy these animals. Many diseases,including bubonic plague, are spread by rodents, and rat control,especially in urban areas, is a major component of public healthefforts. Insects also transmit many diseases (a notable example ismalaria). The spread of insect-borne diseases can be controlled byeliminating breeding areas for insects (for example, draining areaswhere stagnant water collects) and using pesticides. Many importedanimals must be tested for specific diseases to prevent the introductionof those diseases into the country. Vaccination programsMost states now require that parents or guardians show proof ofvaccination before their children can be enrolled in day-care facilitiesor public schools, although some states allow certain exemptions,including exemptions based on religious beliefs. The value ofimmunization for an individual’s health is obvious; however, it is also important for public health. If a certain proportion of a population (called the threshold proportion)is immune to a disease, the pathogen that causes that disease will beunable to reproduce itself at a high enough level to maintain itself inthe population. This is because once the infected host recovers or dies,there will not be enough new, susceptible hosts for the pathogen toinfect. Eventually, the pathogen cannot spread any further and could beeliminated from the population. Even if elimination of the pathogen doesnot occur, there will be relatively few cases of the related diseaseand epidemics of the disease in the population will be avoided. Thisphenomenon is called herd immunity.The threshold proportion varies depending on the disease and otherconditions in the relevant population. Vaccination programs led bypublic health officials aim to achieve the immunization of at least thethreshold number of individuals for the population. Public health organizationsCities and other local areas have public health agencies that enforceregulations, provide public health services such as vaccinationprograms, and monitor and report the incidence of particular diseases tostate and federal agencies. State public health agencies are affiliatedwith laboratories and staff epidemiologists for investigating diseasecases.All of these agencies report data to the U.S. Public Health Service.NIH, the funding agency of this module, began in 1887 as the Laboratoryof Hygiene; NIH is one of eight health agencies of the U.S. PublicHealth Service. It supports health-related research aimed atunderstanding, preventing, treating, and controlling infectious andother diseases of humankind. The Public Health Service also operates theCenters for Disease Control and Prevention (CDC) in Atlanta, Georgia,and the Food and Drug Administration (FDA). CDC staff investigatedisease outbreaks, publish a summary of current epidemiological reports,and sponsor a variety of education programs, research projects, andreference laboratories. FDA monitors the safety of our food, medicines,and many other products that we use daily. Finally, the World HealthOrganization (WHO) provides international surveillance and control ofdisease. Among other efforts, WHO coordinates multinational vaccinationcampaigns. Treatment of Infectious DiseasesWhile literally meaning “destroyer of life,” the term “antibiotic”has become the most commonly used word to refer to a chemical substanceused to treat bacterial infections. The term “antimicrobial” has asomewhat broader connotation, generally referring to anything thatinhibits the growth of microbes. Technically, the term antimicrobialdoes not encompass the “antihelminthic” drugs because worms are notmicroscopically small. Antimicrobials can be either microbistatic(inhibiting the replication of the microbe) or microbicidal (actuallykilling the target microorganism). In the former case, a combination oftherapy and immunity may be required to finally terminate the infection. Treatment of bacterial diseasesBecause bacteria are prokaryotes, it has been relatively easy to findand develop antibacterial drugs that have minimal side effects. Thesedrugs target structural features and metabolic characteristics ofprokaryotes that are significantly different from those in eukaryoticcells. Drugs used to treat bacterial diseases can be grouped intocategories based on their modes of action. In general, these drugsinhibit cell wall synthesis, protein synthesis, nucleic acid synthesis,or other enzyme-catalyzed reactions.The penicillins and cephalosporins all interfere with the synthesisof the peptidoglycan layer in prokaryotic cell walls. Because eukaryoteshave neither the peptidoglycan components nor the enzymes thatsynthesize them, these drugs do not affect the host cells. A secondclass of drugs, including chloramphenicol, the tetracyclines, anderythromycin, bind to prokaryotic ribosomes and inhibit proteinsynthesis. Prokaryotic ribosomes are structurally different fromeukaryotic ribosomes, so these drugs have minimal effect on eukaryoticcells. Nevertheless, some of them may be toxic to some human tissueswhen they are used in high doses or for prolonged periods of time.Rifampicin is one of the antibiotics frequently used for treatingtuberculosis. This drug inhibits prokaryotic RNA synthesis. DNAsynthesis in prokaryotes may be inhibited by the fluoroquinolones. Incontrast, the sulfonamides stop bacterial infections by inhibiting otherenzymes. Sulfonamides interfere with the synthesis of folic acid, avitamin necessary for nucleic acid synthesis. Most bacteria mustsynthesize their own folic acid because their membranes are impermeableto external folic acid. Mammalian cells are not affected by sulfonamidesbecause they are unable to make their own folic acid and have evolvedmechanisms for transporting external folic acid across their membranes. Treatment of viral diseasesIn general, drugs that effectively inhibit viral infections arehighly toxic to host cells because viruses use the host’s metabolicenzymes in their reproduction. For this reason, most illnesses due toviruses are treated symptomatically until the host’s immune systemcontrols and eliminates the pathogen (or the host dies). Antiviral drugsthat are used typically target virus-specific enzymes involved in viralnucleic acid synthesis. One of the most familiar of these drugs isacyclovir, which is used to treat outbreaks of genital herpes.Amantadine is an antiviral drug sometimes used to prevent or moderateinfluenza among those at high risk of severe illness from the disease.In addition to antiviral drugs that inhibit the replication of the HIVgenome (such as AZT), AIDS patients today are also prescribed proteasesthat interfere with the packaging of the HIV genome into virusparticles. Treatment of fungal and parasitic diseasesThe development of drugs to treat fungal, protozoan, and helminthicdiseases is challenging because agents that kill or inhibit the growthof these eukaryotic organisms are also highly toxic to mammalian cells.Because fungi and protozoa are rapidly proliferating cells, drugsagainst these organisms tend to target key components of theirreplicative or biosynthetic pathways. Common antifungals inhibit sterolsyntheses (the azole derivatives) or disrupt the cell membrane (polyeneslike amphotericin B). Most antihelminthic drugs target adult worms,which are no longer growing and do not replicate. These drugs are oftenaimed at inhibiting fundamental processes, such as energy production andmuscle function (for example, the benzimidazoles and avermectins), orat targets involved in egg production or larval development.Malaria, a protozoan disease, was successfully treated for many years with chloroquine. In recent decades, Plasmodiumspecies that are resistant to this drug have appeared and spread toareas where malaria is a common threat. In those areas, a combination ofthe drugs sulfonamide and pyrimethamine is frequently used to treat thedisease. Resistance to antimicrobial agentsOne of the ongoing problems scientists and medical workers face inthe fight against infectious diseases is the development of resistanceto the agents used to control them. The phenomenon of resistance hasbeen known since almost the beginning of antibiotic use. For example,penicillin was introduced for clinical use in treating bacterialinfections in the 1940s. As early as 1943, Alexander Fleming, thediscoverer of penicillin, observed that some bacteria were resistant tothe drug and warned that indiscriminate use of penicillin would lead tothe proliferation of resistant pathogenic bacteria. By 1946, medicalstaff at a London hospital estimated that 14 percent of thestaphylococcal strains isolated from their patients were resistant topenicillin. Today, more than 90 percent of these bacteria are resistant.In an environment of widespread penicillin use, selection for resistantbacteria occurred; that is, the pathogenic organisms evolved.The same process has occurred for many other antimicrobial drugs.Alarmingly, many pathogens are simultaneously acquiring resistance tomultiple drugs. For example, some strains of Mycobacterium tuberculosis are resistant to all of the currently available drugs used for treatment. Mechanisms of antimicrobial resistanceAntibiotic resistance appears as a result of changes in genes or theacquisition of genes that allow the pathogen to evade the action ofantimicrobial drugs. Resistance mechanisms include structural changes inor around the target molecule that inhibit the drugs’ ability to bindto it; reduced permeability of the cell membrane to the drug, activelypumping the drug out of the cell after it has entered; and production ofenzymes that inactivate the antibiotic after it has been taken up bythe cell. Microbes that produce larger than normal amounts of the targetmolecule may be “less susceptible” (as opposed to resistant) to a drug,meaning it takes a higher drug level to adversely affect that microbe. Transfer of antimicrobial-resistance genesBacteria have many methods for developing resistance. Antibioticresistance initially arises as mutations to existing genes; however,many (probably most) bacteria acquire these genes rather thanexperience the mutation themselves. Resistance genes are transferred toother members of the same species and across species by a variety ofbacterial genetic exchange mechanisms. Many gram-negative bacteria,including Escherichia coli and Salmonella species, can transfer extra-chromosomal genetic material called plasmids via the process of conjugation.Bacteria endowed with the plasmids have numerous pili along theirsurfaces; one of these extends to a plasmid-lacking bacterium as aconjugation tube. The plasmid then replicates, and one copy travelsthrough the conjugation tube into the recipient bacterium. One largeclass of plasmids is called resistance plasmids because they carry genesthat confer antibiotic resistance. Many resistance plasmids carry genesfor resistance to multiple antibiotics; thus, one conjugation event cansimultaneously transfer resistance to several antibiotics.Some species of bacteria are capable of taking up free-floating bits of DNA from their environments in a process known as bacterial transformation.If they take up a DNA fragment containing an antibiotic resistancegene, they may become resistant to that antibiotic. Another mechanism ofgenetic exchange in bacteria is transduction. Bacteria aresubject to viral infection. When a bacteria cell is infected, the virustakes over the cell’s metabolism, directing synthesis of its geneticmaterial and production of the components of the viral particle.Simultaneously, the host bacterial DNA is degraded. In the last stage ofvirus production, its genetic material is encapsulated in a proteincoat. Occasionally, a piece of the host bacterial DNA may be packaged ina viral particle. The resulting “transducing particle,” like a normalviral particle, has the ability to attach to a recipient bacterium andtransfer its genetic material into the cell. However, in this case, thetransferred genetic material may be a bacterial gene that pro videsresistance to an antibiotic.Finally, many transposons carry antibioticresistance genes.Transposons are sequences of DNA that are capable of insertingthemselves randomly into genomes. Because they do not appear to rely onspecific genetic sequences of the target insertion site, they canreadily move across species.Although mutations that result in antibiotic resistance and, less so,bacterial genetic exchange, are rare events, they need occur only once.In an environment of heavy antibiotic use, the forces of naturalselection will favor the propagation of resistant variants of apathogen. The human body is a rich environment for the growth of largenumbers of bacteria and for the interaction of a variety of pathogenicand nonpathogenic bacteria. Thus, there is optimal opportunity for raremutational and genetic exchange events.Other pathogens have more limited options for drug resistance.Strains of pathogens develop that are naturally less susceptible to aparticular drug due to a normally occurring mutation. In the face ofcontinuing drug use, this strain rapidly grows out of the populationbeing spread through the usual transmission process. Malaria, aprotozoan disease, was successfully treated for many years withchloroquine, a drug that was widely available over the counter inregions where malaria was a problem. In recent decades, Plasmodium strains that are resistant to this drug have appeared and spread throughout Africa, South America, and Southeast Asia. Emerging and Re-emerging Infectious DiseasesFifty years ago many people believed the age-old battle of humansagainst infectious disease was virtually over, with humankind thewinners. The events of the past two decades have shown the foolhardinessof that position. At least a dozen “new” diseases have been identified(such as AIDS, Legionnaire disease, and hantavirus pulmonary syndrome),and traditional diseases that appeared to be “on their way out” (such asmalaria and tuberculosis) are resurging. Globally, infectious diseasesremain the leading cause of death, and they are the third leading causeof death in the United States. Clearly, the battle has not been won. Emerging infectious diseases are diseases that (1) have notoccurred in humans before (this type of emergence is difficult toestablish and is probably rare); (2) have occurred previously butaffected only small numbers of people in isolated places (AIDS and Ebolahemorrhagic fever are examples); or (3) have occurred throughout humanhistory but have only recently been recognized as distinct diseases dueto an infectious agent (Lyme disease and gastric ulcers are examples). Figure 7 lists several examples of infectious diseases that have emerged in the last three decades.A review of Figure 7reveals that environmental changes are related to the emergence of manyinfectious diseases. For example, Lyme disease, hantavirus pulmonarysyndrome (HPS), and Lassa fever all emerged when humans beganencountering the insect vector (for Lyme disease) or rodent host (forHPS and Lassa fever) of the causative agents in greater numbers thanever before. Factors related to the emergence of infectious diseasessuch as Legionnaire disease and hemolytic uremic syndrome includechanging technologies: air conditioning systems for the former diseaseand mass food production for the latter. Re-emerging infectious diseases are diseases that once were majorhealth problems globally or in a particular country, and then declineddramatically, but are again becoming health problems for a significantproportion of the population (malaria and tuberculosis are examples).Many specialists in infectious diseases include re-emerging diseases as asubcategory of emerging diseases. Figure 8 lists examples of re-emerging infectious diseases.A review of Figure 8reveals some explanations for the re-emergence of infectious diseases.Tuberculosis has re-emerged due to evolution of the causative bacteria.The pathogen has acquired resistance to the antibiotics used to treattuberculosis (either through mutation or genetic exchange) and thelong-term use of antibiotics (both within one individual and across thepopulation) has selected for the pathogen’s proliferation. Malaria hasalso become drug resistant, and the vector mosquito has acquiredresistance to pesticides as well. The re-emergence of diseases such asdiphtheria and whooping cough (pertussis) is related to inadequatevaccination of the population. When the proportion of immune individualsin a population drops below a particular threshold, introduction of thepathogen into the population leads to an outbreak of the disease.Despite the challenges of emerging and re-emerging infectiousdiseases, the results of basic research, such as that sponsored by NIH,show that there is reason for hope. AIDS was first described in 1981,and it took two years to identify the retrovirus that causes AIDS, whichwas named the human immunodeficiency virus. In contrast, less than fourmonths elapsed between the description of hantavirus pulmonary syndrome(HPS) in 1993 and the identification of the previously unknown viralagent, now called Sin Nombre virus. One difference between these twocases is that the years that intervened between the advent of AIDS andthe advent of HPS saw the development of polymerase chain reaction, apowerful new research technique that allows rapid identification ofcausative agents. Recommendations for avoiding and/or treating of newinfectious diseases become possible when new techniques, developedthrough basic research, are applied to the problem of disease emergence.Other examples of the benefits of basic research include thedevelopment of HIV protease inhibitors by researchers funded by NIH andothers. These drugs, when used in combination with other anti-HIV drugs,are responsible for the dramatic decrease in deaths from AIDS in theUnited States. One active area of research at NIH is the development ofnew types of vaccines based on our new understanding of the immunesystem. In addition, basic research on the immune system and hostpathogen interactions has revealed new points at which vaccines couldwork to prevent diseases.Finally, basic research on the ecology of disease organisms—theirreservoirs, modes of transmission, and vectors, if any—reveals points atwhich preventive measures can be used to interrupt this cycle andprevent the spread of disease. For example, research supported by NIAIDdelineated the mechanism of Lyme disease transmission and how diseaseresults: The tick vector was identified and the life cycle of thecausative bacterium was traced through deer and rodent hosts.Understanding this ecology has led to predictions about the regionswhere and years when the threat of Lyme disease is greatest, as well asrecommendations to the public for avoiding infection. These examples andothers demonstrate that investment in basic research has greatlong-term payoffs in the battle against infectious diseases. Infectious Diseases and SocietyWhat are the implications of using science to improve personal andpublic health in a pluralist society? As noted earlier, one of theobjectives of this module is to convey to students the relationshipbetween basic biomedical research and the improvement of personal andpublic health. One way to address this question is by attending to theethical and public policy issues raised by our understanding andtreatment of infectious diseases. Ethics is the study of good and bad, right and wrong. It has todo with the actions and character of individuals, families, communities,institutions, and societies. During the last two and one-halfmillennia, Western philosophy has developed a variety of powerfulmethods and a reliable set of concepts and technical terms for studyingand talking about the ethical life. Generally speaking, we apply theterms “right” and “good” to those actions and qualities that foster theinterests of individuals, families, communities, institutions, andsociety. Here, an “interest” refers to a participant’s share orparticipation in a situation. The terms “wrong” or “bad” apply to thoseactions and qualities that impair interests.Ethical considerations are complex, multifaceted, and raise manyquestions. Often, there are competing, well-reasoned answers toquestions about what is right and wrong, and good and bad about anindividual’s or group’s conduct or actions. Thus, although science hasdeveloped vaccines against many diseases, and public health lawsencourage their widespread use, individuals are permitted (in most, butnot all, states) to choose not to be vaccinated.Typically, answers to these questions all involve an appeal to values. A valueis something that has significance or worth in a given situation. Oneof the exciting events to witness in any discussion in ethics in apluralist society is the varying ways in which the individuals involvedassign value to things, persons, and states of affairs. Examples ofvalues that students may appeal to in discussions of ethical issuesinclude autonomy, freedom, privacy, protecting another from harm,promoting another’s good, justice, fairness, economic stability,relationships, scientific knowledge, and technological progress.Acknowledging the complex, multifaceted nature of ethical discussionsis not to suggest that “anything goes.” Experts generally agree on thefollowing features of ethics. First, ethics is a process of rationalinquiry. It involves posing clearly formulated questions and seekingwell-reasoned answers to those questions. For example, developingcountries suffer particularly severely from many infectious diseasesbecause conditions of crowding and poor sanitation are ideal for thegrowth and spread of pathogens. The same is true for many inner cityenvironments. These places provide a constant reservoir ofdisease-causing agents. We can ask questions about what constitutes anappropriate ethical standard for allocating health care funds forcurtailing the spread of infectious diseases. Should we expend publicresearch dollars to develop drugs whose cost will be out of reach fordeveloping countries or those in the inner cities? Is there any legaland ethical way for the United States to prevent over-the-counter salesof antibiotics in other countries, a practice that may enhance theevolution of antibiotic resistant pathogens? Well-reasoned answers toethical questions constitute arguments. Ethical analysis and argument, then, result from successful ethical inquiry.Second, ethics requires a solid foundation of information andrigorous interpretation of that information. For example, one must have asolid understanding of infectious disease to discuss the ethics ofrequiring immunizations and reporting of infectious diseases. Ethics isnot strictly a theoretical discipline but is concerned in vital wayswith practical matters. This is especially true in a pluralist society.Third, because tradeoffs among interests are complex, constantlychanging, and sometimes uncertain, discussions of ethical questionsoften lead to very different answers to questions about what is rightand wrong and good and bad. For example, we acknowledge that individualshave a right to privacy regarding their infectious disease status. Yet,some argue that AIDS patients who knowingly infect others may havetheir right to privacy overridden so that partners may be notified ofthe risk of contracting AIDS.It is our hope that completing the activities in this module willhelp students see how understanding science can help individuals andsociety make reasoned decisions about issues relating to infectiousdiseases and health. Science provides evidence that can be used tosupport ways of understanding and treating human disease, illness,deformity, and dysfunction. But the relationships between scientificinformation and human choices, and between choices and behaviors, arenot linear. Human choice allows individuals to choose against soundknowledge, and choice does not necessarily lead to particular actions.Nevertheless, it is increasingly difficult for most of us to deny theclaims of science. We are continually presented with great amounts ofrelevant scientific and medical knowledge that is publicly accessible.As a consequence, we can think about the relationships among knowledge,choice, behavior, and human welfare in the following ways:One of the goals of this module is to encourage students to think in terms of these relationships, now and as they grow older. Do you need a similar assignment done for you from scratch? We have qualified writers to help you. 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