learning objectives - university of phoenix

Infectious DiseasesC H A P T E R

Learning Objectives

After studying this chapter, you should be able to

■ Define infectious diseases

■ Describe and compare the characteristics of bacteria, viruses, fungi,helminths, and arthropod vectors

■ Explain the chief ways that infectious diseases are transmitted

■ Explain the kinds of treatment for bacterial, viral, fungal, andparasitic infectious diseases

■ Describe how vaccines work and the names and uses of commonvaccines

■ Understand the appropriate use of antibiotics and explain theproblem of antibiotic resistance

■ Describe examples and the causes of emerging infectious diseases

3Thanks to modern medicine,notorious infectious diseaseslike tuberculosis are things ofthe past.

Fiction:Antibiotic resistance,chronic disease, environmentalchanges, and other factors areresponsible for the re-emer-gence of many infectious dis-eases, including tuberculosis.

Fact orFiction?

Kaposi’s sarcoma. (Courtesy ofthe CDC/Dr. Edwin P. Ewing,Jr., 1979.)

Human Diseases: A Systematic Approach, Sixth Edition, by Mary Lou Mulvihill, Mark Zelman, Paul Holdaway, Elaine Tompary, and Jill Raymond. Copyright © 2006 by PearsonEducation, Inc.

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Use the web address to the left to access the free,interactive Companion Website created for this textbook.It features chapter-specific exercises, Internet links,news links, and an audio glossary. Additionally, explorethe CD-ROM that accompanies this book to discoverDisease Focus videos and a rich array of activities thataccompany this chapter.

MedMediawww.prenhall.com/mulvihill

West Nile Virus

Before 1999, the American public had never heard of WestNile Virus; indeed, microbiologists had never detected thedisease in the western hemisphere. In New York that springand summer, tens of thousands of birds died suddenly and in

the open: crows, blue jays, chickadees, and prized birds of the BronxZoo fell. Horses, which are susceptible to related encephalitisviruses, showed signs of infection. Humans too became sick, andsome died. Gene testing soon demonstrated the cause to be WestNile Virus, named for the West Nile region of Uganda where it wasfirst discovered. Transmitted by Culex mosquitoes and carried by in-fected birds, the virus efficiently swept across America. Fortunately,very few mosquitoes carry the virus. Of those people bitten, few be-come sick and fewer than 1%, mostly elderly and immune-compromised, become critically ill. Control measures, such as spray-ing and use of DEET repellents, appear to be effective in curtailingmosquito populations and preventing infection. Data suggest that,following the first waves of infection, some bird populations have ac-quired immunity to the virus.

Disease Chronicle


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30 ■ Chapter Three Infectious Diseases

Introduction

What is the impact of infectious disease onhuman health? According to the World HealthOrganization, of 52 million deaths occurringworldwide each year, about 17 million arecaused by infectious diseases. Of course, manymore than 17 million people become ill eachyear with an infectious disease, and that suffer-ing remains uncounted. As the world popula-tion grows, infectious disease will continue togrow in importance.

Some infectious diseases described in thisbook have been in existence for a long time,whereas others have emerged as new pathol-ogy. It is important to have a framework for un-derstanding these infectious diseases. Thischapter describes the nature of infectious dis-eases, surveys the types of microorganisms re-sponsible for infections, explains their trans-mission, and outlines treatment and controlstrategies.

Infectious diseases are those diseases causedby pathogenic microorganisms. These diseasesmay be transmitted to humans by other hu-mans or by some other element in the environ-ment. Those diseases transmitted from humanto human are said to be contagious or communic-able. Measles and influenza are well-known con-tagious diseases. Infectious diseases are classi-fied as noncommunicable if they cannot be trans-mitted directly from person to person. For ex-ample, rabies can be transmitted by the bite ofa rabid raccoon, and cholera is transmitted bydrinking fecal-contaminated water. In any case,some type of pathogenic microorganism causesthese diseases.

Pathogenic Microorganisms

Most microorganisms do not cause disease.Those that do cause disease are called pathogens.Humans can be infected by a variety ofpathogens, ranging from tiny, single-celled bacte-ria to macroscopic, complex worms (Figure 3–1).

BacteriaBacteria are microscopic, single-celled organ-isms. A simple structure (no nucleus or mem-branous organelles) and small size (1 to 10 μm)are key characteristics that differentiate bacteriafrom other single-celled organisms. Althoughoften described as simple, they are far fromprimitive, for they have adapted to a wide varietyof habitats and have evolved complex strategiesfor infecting and surviving in the human body.

Bacteria have cell walls, a rigid layer of organicmaterial surrounding their delicate cell mem-branes. These walls give bacteria their character-istic shapes. Bacteria may have spherical, roundcells called cocci, rod-shaped cells called bacilli,spiral-shaped cells called spirilla, corkscrew-shaped cells called spirochetes, or comma-shapedcells called vibrios. Figure 3–1 shows the cellstructure of a rod-shaped bacterium. The wallsprotect these cells; should walls be disrupted,cells are susceptible to bursting. This is the ac-tion of the antibiotic penicillin. Penicillin inter-feres with correct cell wall construction of certaintypes of bacteria. The bacteria cell walls may bethick, thin, or absent. The thickness and chemi-cal composition of the cell wall accounts for theway certain cells stain during the gram stain pro-cedure. During the gram stain, thick-walled cellsturn purple and thin-walled cells become red;thus, bacteria can be identified using this tech-nique (Figure 3–2). Identification is critical to ob-tain an accurate diagnosis and effective treat-ment of an infection. Table 3–1 lists commongram-positive and gram-negative pathogens andthe diseases associated with them. Other bacte-ria that do not fit into the above categories ofshape and gram stain properties include thechlamydias and rickettsias, which are intracellu-lar parasites. Chlamydia trachomatis causes asexually transmitted infection. Rickettsias aretransmitted by ticks and cause diseases such astyphus and Rocky Mountain spotted fever.

Bacteria grow rapidly and reproduce by split-ting in half, a process known as binary fission.Under favorable conditions, this process maytake only 30 minutes, which means that a smallnumber of cells may increase to a very largenumber in a relatively short time. The reproduc-

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Chapter Three Infectious Diseases ■ 31

Capsid(protein coat)

Membranousenvelope

Flagellum

Liver fluke(Fasciola hepatica)

Tapeworm(Taenia)

Roundworm(Ascaris)

Spikes(protein)

Cytoplasm

Capsuleor slimelayer

Cell wall

Cell membrane

Amoeba;causes amoebicdysentery

Trypanosoma;causes sleepingsickness

Plasmodium(in red blood cell);causes malaria

Chromosome

Ribosomes

InclusionNucleic acid(DNA or RNA)

0.1 μm

1 cm

1 μm

10 μm

A. Bacterium

C. Protozoan pathogens D. Multicellular parasites

B. Virus

Figure 3–1 Types of pathogenic organisms include bacteria (A), viruses (B), protozoa (C), and helminths, or worms (D).

A. B.

Figure 3–2 Gram-stained bacteria on microscope slide. (A) Pink rod-shaped cells are Escherichia coli., (Courtesy of the CDC,1979.) (B) Purple cocci are Staphylococcus aureus. (© SIU BioMed/Custom Medical Stock Photo.)


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Bacterial Pathogens Disease(s)

Gram-positive cocci

Staphylococcus aureus Skin infections, food poisoning

Streptococcus pyogenes Pharyngitis (strep throat)

Streptococcus pneumoniae Lobar pneumonia

Gram-positive bacilli

Clostridium tetani Tetanus

Clostridium botulinum Botulism

Bacillus anthracis Anthrax

Gram-negative cocci

Neisseria gonorrhoeae Gonorrhea

Gram-negative bacilli

Salmonella typhimurium Salmonellosis

Legionella pneumophila Legionellosis (Legionnaires’ disease)

Pseudomonas aeurginosa Urinary tract infections, burn infections

Spirilla

Campylobacter jejuni Acute enteritis, diarrhea

Helicobacter pylori Gastritis, peptic ulcer

Spirochetes

Treponema pallidum Syphilis

Borrelia burgdorferi Lyme disease

Vibrios

Vibrio cholerae Cholera

Chlamydias and rickettsias

Chlamydia trachomatis Trachoma, sexually transmitted chlamydia

Rickettsia prowazekii Typhus

Rickettsia rickettsii Rocky mountain spotted fever

Table 3–1 Common Bacterial Pathogens and Associated Diseases


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tion of a cell and its genetic material occurs withvery few errors or mutations. Still, this rapidgrowth rate virtually guarantees that mutationswill arise, and some of these will favor survival ofthe bacteria under certain conditions.

Some bacteria produce endospores, commonlycalled spores. The endospore contains the geneticmaterial of the cell packaged in a tough outer coatthat is resistant to desication, acid, extreme tem-perature, and even radiation. Endospores germi-nate and form growing cells when conditions arecorrect. Certain diseases, like tetanus and botu-lism, can be caused by endospores that contami-nate food, water, or wounds.

Bacteria cause illness in humans in a varietyof ways. A particularly potent toxin called endo-toxin causes life-threatening shock. This toxin isreleased into tissues when gram-negative cellsdie. Some bacteria produce other types of toxinsthat interfere with normal physiology. For ex-ample, tetanus is caused by the toxin producedby the bacterium Clostridium tetani. The tetanustoxin interferes with the ability of muscle cells

to relax, resulting in frozen, rigid muscles char-acteristic of the disease. Other toxins are en-zymes that enable the bacteria to spreadthrough tissues and to obtain nutrients. Somesigns and symptoms of bacterial infection arenot generated by the bacteria themselves butrather by the immune response to the infection.Common characteristics of certain bacterial in-fections include swelling, redness, pain, fever,and pus.

VirusesViruses are infectious particles made of a core ofgenetic material (either RNA or DNA) wrapped ina protein coat (capsid). Some viruses also have alipid membrane surrounding their capsid(Figure 3–3). Viruses are not considered livingorganisms because they do not independentlygrow, metabolize, or reproduce. Viruses mustcarry out their life processes by entering cellsand directing the cells’ energy, materials, andorganelles for these purposes.

▲ Bacterial infection has caused the air space tofill with pus and fluid. View from front. (Courtesyof the CDC.)

SIDE by SIDE ■

▼ ▼Pneumonia

▲ Normal chest x-ray showing clear lungs. Viewfrom right side. (Courtesy of the CDC.)


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Nucleicacid

Capsomere

Enveloped virion

Envelope

Spikes

Figure 3–3 Structure of an enveloped virus.

Certain viruses infect and grow in only certaintypes of human cells. Some viruses, like coldviruses, target only cells of the respiratory ep-ithelium. Others, like herpes viruses, reproducein nervous tissue. Signs and symptoms of infec-tion result from the way these viruses reproduce

in cells or from the way the immune system re-sponds to viral infection. Some viruses cause thecells they infect to lyse, or rupture. This is thecase when HIV infects and reproduces within Tcells. The resultant T-cell deficiency leads to theimmunodeficiency in AIDS. Other viruses sus-tain a latent infection, whereby the viruses insertthemselves in cells and do not reproduce. At thistime, no symptoms may be present. Later, a trig-ger such as stress, infection with anotherpathogen, or a weakened immune defense, acti-vates the viruses. Symptoms of the disease thenmanifest themselves as the viruses reproduce.This pattern is seen in the waxing and waning ofherpes infections. Other viruses cause abnormalcell growth because the viral genetic material in-terferes with the cell’s growth-control genes.Abnormal growth of tissues is discussed furtherin Chapter 4. Such growth may be benign, as ina dermal wart. However, the result may be a ma-lignant growth, that is, a cancer. For example,human papillomavirus infection is linked to cer-vical cancer. Table 3–2 lists examples of virusesand their associated diseases.

Viruses Diseases

DNA viruses

Herpes viruses

Herpes simplex 1 Cold sores and fever blisters

Herpes simplex 2 Genital herpes

Varicella-Zoster Chicken pox and shingles

Hepatitis B Hepatitis (“serum hepatitis”)

Epstein-Barr Infectious mononucleosis

RNA viruses

Influenza A, B, C Influenza (“flu”)

Hepatitis A, C, D, E Infectious hepatitis

Rhinovirus Common cold

Human immunodeficiency virus HIV infection/AIDS

Table 3–2 Viral Pathogens and their Diseases


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ProtozoaProtozoa are single-celled eukaryotic microor-ganisms. They are much larger than bacteriaand have complex internal structures, includinga nucleus and membranous organelles.Protozoa are found in nearly every habitat, andmost do not cause disease. Protozoa are classi-fied as amoeboids, flagellates, ciliates, andsporozoans.

Amoeboids move by means of cell membraneextensions called pseudopodia. These extensionsmay be familiar in another context—in humanphagocytic leukocytes, which use pseudopodiato crawl about and ingest particles. An amoebaof great health concern is Entamoeba histolyt-ica, the cause of amoebic dysentery (Chapter10), an intestinal infection acquired from feces-contaminated food or water. The flagellatesswim by using one or more whiplike appendagescalled flagella. Pathogens in this group includeTrypanosoma, the cause of African sleepingsickness, and Giardia, the cause of giardiasis,an intestinal infection. Ciliates move by meansof numerous short, hairlike projections calledcilia. There are few pathogens among the cili-ates. Sporozoans are not mobile. Plasmodium isthe most notorious among them because itcauses malaria. This diverse group of microor-ganisms causes disease in a variety of ways.Protozoa may invade and destroy certain tis-sues, or they may provoke damaging inflamma-tory responses.

FungiFungi are single-celled or multicelled organismswith cell walls that contain a special polysac-charide called chitin. Fungi use specialized fila-ments called mycelia to absorb nutrients fromtheir surroundings. They also have reproductivestructures bearing spores, which are known al-lergens.

Fungal infections are known as mycoses.Healthy human tissue is relatively resistant tofungal infections but may be susceptible undercertain circumstances. Fungi can more easilyinfect damaged tissue than intact healthy tis-sue. Also, immunocompromised hosts may beunable to resist fungal infections. Fungi cause

disease by producing toxins, interfering withnormal organ structure or function, or inducinginflammation or allergy. Some common fungaldiseases include candidiasis, an infection ofskin or mucous membranes caused by the yeastCandida. Histoplasmosis is a respiratory infec-tion caused by Histoplasma, which is inhaled indust from soil contaminated with bird drop-pings. Microsporum and Trichophyton cause avariety of ringworm infections of the skin, hair,and nails (Chapter 17).

HelminthsThe wormlike animals, which include round-worms and flatworms, are called helminths.Like other animals, helminths are complex mul-ticellular motile organisms. They often havewell-developed reproductive systems capable ofproducing large numbers of offspring. Manyhelminths have also evolved complex life cyclesand strategies for infecting new hosts. In-fections with these organisms are often calledinfestations.

Roundworms are relatively round in cross-sec-tion. They include filarial (threadlike) wormsthat infect the lymphatic system, like Wucheria,the cause of elephantiasis. The human hook-worm Necator (Figure 3–4) infects the small in-testines, whereas the pinworm Enterobius in-fects the large intestine.

Flatworms, as the name suggests, have flat-tened bodies. Schistosoma, a type of helminthcalled a fluke, causes schistosomiasis, an infec-tion of blood vessels. This infection is found inSoutheast Asia, Africa, and parts of SouthAmerica. Tapeworms like Taenia infect the in-testines.

In some cases, transmission of helminths isrelatively straightforward: Ascaris eggs can beswallowed in feces-contaminated food or water.In other cases, many pieces of a complex lifecycle need to be in place to sustain infections.For example, although the juvenile Schistosomacan infect a person simply by swimming to ahuman foot and burrowing through the skin,the complete life cycle of Schistosoma dependson a particular snail as an intermediate host fordevelopment of other immature stages of theworm. The water in which the snails grow needs


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to be still and needs also to be contaminatedwith infested human urine or feces. Otherhelminths are transmitted by arthropod vectors,which are discussed later in this chapter.

Helminths cause disease by using the host’snutrients, as pinworms and tapeworms dowhen they infest the intestines. Others feed onblood, causing anemia. Ascaris can block orperforate the intestines when infestations be-come large, or the eggs or worms themselvescan induce severe inflammatory responses.

Arthropod VectorsAnimals with jointed legs and a hard exoskele-ton are called arthropods. These include ticks,mites, lice, flies, mosquitoes, and fleas. Somearthropods act as disease vectors. That is, theanimals transmit pathogenic microorganisms tohumans. An arthropod may transmit apathogen when biting and feeding on humanblood. The Anopheles mosquito transmits

Plasmodium when it feeds on an infected humanand carries the malaria parasite to anotherhuman. Some arthropods simply carry pathogenson their bodies. The housefly Musca can carrybacteria on its feet. In many cases, the arthro-pod is an essential part of the pathogen’s lifecycle. Humans will not get infected with thepathogen unless the appropriate arthropod vec-tor is present. Therefore, it is very important toidentify these arthropods and to understandtheir lives if the diseases they carry are to becontrolled.

Other animals are also important for infec-tious diseases. Some animals act as reservoirs ofinfection, acting as sources of the pathogen andpotential sources of disease. Certain hoofed an-imals and domestic cattle are reservoirs forAfrican sleeping sickness. Deer and mice cancarry the spirochete and harbor the tick thattransmits Lyme disease. Still other animals arethe only means by which a disease is transmit-ted. Rabies viruses are transmitted by the biteof infected raccoons, foxes, bats, and domesticdogs.

Infection Prevention and Control

Effective prevention and control of infectiousdiseases requires knowledge of the nature of thepathogens and their means of transmission.

Transmission and EpidemiologyEpidemiology is the study of the transmission,occurrence, distribution, and control of disease.Infectious diseases can be transmitted directlyfrom an infected human to a susceptible human.This route is called horizontal transmission.Influenza, gonorrhea, measles, and many otherdiseases described elsewhere in this book aretransmitted this way. Other infectious diseasesare transmitted from one generation to the next,as when syphilis, HIV/AIDS, or ophthalmianeonatorum (an eye infection; see Chapter 12)are transmitted to newborns from infected moth-ers. This route is called vertical transmission.

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Swallowed

Attached to the small intestine

Adults the small intestine

Eggs in feces

Penetrates skin

Circulation

Lungs

Trachea

Pharnyx

(diagnostic stage)

Rhabditiform larva hatches

Filariform larva(infective stage)

Rhabditiform larva in soil

(occasionally in old feces)

Hookworm

MAN

EXTERNAL ENVIRONMENT

Figure 3–4 The human hookworm lifecycle.


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Humans transmit pathogens in respiratorydroplets, blood, semen, feces, urine, andthrough direct physical contact. Measles, pneu-monia, tuberculosis, and influenza are well-known diseases transmitted in respiratorydroplets generated by coughing and sneezing.HIV and hepatitis B and C are viruses transmit-ted in blood and blood products. Sexually trans-mitted diseases like gonorrhea, chlamydia, andHIV are transmitted in semen and vaginal se-cretions. Food poisoning, like salmonellosis,and dysentery are transmitted by consumptionof fecal-contaminated food and water (seeChapter 10 for salmonella and dysentery).

In some cases, humans are not involved in di-rect transmission of a disease. For example, aneffective campaign to control malaria would re-quire control of the parasite’s mosquito vector.

Study of the distribution and frequency ofdiseases can help predict and prevent disease.The number of new cases of a disease in a pop-ulation is its incidence. The incidence of somediseases follows a pattern, as when influenzaincidence increases in winter and subsides insummer, or when Lyme disease increases insummer and subsides in winter. The number ofexisting cases of a disease is known as itsprevalence, and this information can tell howsignificant the disease is to a certain popula-tion. When a disease always occurs at low levelsin a population, it is said to be endemic. If a dis-ease occurs in unusually large numbers over aspecific area, it is said to be epidemic. Influenzaoccurs as epidemics. When an epidemic hasspread to include several large areas worldwide,it is said to be pandemic. AIDS is considered tobe pandemic. When a disease suddenly occurs

in unexpected numbers in a limited area andthen subsides, this is described as an outbreak.

Certain diseases are under constant surveil-lance in the United States. Such diseases arecalled notifiable diseases. Physicians are requiredto report the occurrence of these diseases to theCenters for Disease Control and Prevention. Thisensures tracking and identification of disease oc-currence and patterns. Chlamydia infectionswere not notifiable before 1998. After these in-fections were classified as notifiable, it was dis-covered that the number of chlamydial infectionssurpassed gonorrhea. Other notifiable infectiousdiseases include measles, mumps, polio, tuber-culosis, Legionnares’ disease, and tetanus.

Control of infectious diseases can be achievedby preventing transmission and by treating in-fected persons. Isolation of infected persons inhospitals and self-imposed isolation, such aswhen a person with influenza remains home inbed, can be effective. Quarantine is the separa-tion of persons who may or may not be infectedfrom healthy people until the period of infec-tious risk is passed. Disinfection of potentiallyinfectious materials is necessary to preventtransmission. Medical and dental implementsneed disinfection to remove human pathogensafter use. The Centers for Disease Control andPrevention recommends Standard Precautions beemployed by all medical personnel during pa-tient care. These precautions are used in all sit-uations, whether a patient is infected or not,and include the use of gloves, correct disposal ofbodily fluids, needles, and other waste.Sanitation techniques that remove infectioussewage from drinking and bathing water andprevent sewage from contaminating food can

Advice for TravelersTraveling out of the country? Have you considered how you will protect yourself and your family from infectiousdiseases? In a different country, you will be faced with different infectious diseases to which you are unlikely to beimmune. Will you need vaccinations or prophylactic medications? The Centers for Disease Control and Prevention inAtlanta, Georgia, maintains a Web site to advise travelers. Before you book your flight, go to www.cdc.gov for the latestinformation on infectious diseases at your destination.

Prevention PLUS!


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dramatically reduce the incidence of choleraand dysentery. The treatment of infected per-sons is a critical component of prevention.

Treatment of Infectious DiseaseThe effective treatment of an infectious diseasedepends on the type of causative pathogen.Bacterial infections can be treated with a vari-ety of antibiotics. The target of some antibioticsis the unique bacterial cell wall. Penicillin andrelated drugs act on the cell wall, and they areespecially useful in controlling gram-positivebacteria. Some antibiotics target the bacterialcell membrane, causing lysis. Other antibioticstarget the protein synthesis machinery of thecell. This is effective because the ribosomes andenzymes involved in bacterial protein synthesisare sufficiently different from those in humancells. Other antibiotics interfere with bacterialmetabolism or with DNA and RNA synthesis.Antibiotic resistance plays an important role inthe increased incidence of bacterial infections.

Correct use of antibiotics can prevent the de-velopment of antibiotic resistance. Resistancearises when bacteria adapt to antibiotics andthe adaptation becomes common in the bacter-ial population, soon rendering the antibioticsineffective. An antibiotic should be used only forbacterial infections. A number of infections, likeinfluenza and the common cold, are viral andare not treatable with antibiotics. Some viral ail-ments closely mimic bacterial infections. For ex-ample, it turns out that group A streptococcicause only 15% of pharyngitis cases, so only asmall proportion of sore throats are really strepthroat. Most sore throats are really caused byviral infections, some of which closely mimicstrep throat because the symptoms includeswelling and exudate. If antibiotics are used formany of these viral infections, bacterial popula-tions will more likely evolve resistance to thoseantibiotics. The Centers for Disease Control andPrevention has asked physicians to take athroat swab and perform the rapid strep anti-gen test to confirm the presence of group Astreptococci before prescribing antibiotics. Ifprescribed, the appropriate antibiotic must beselected. Guidelines are shared and updated for

physicians so that the most effective types anddosages are used for certain infections. Patientsneed to follow through on the prescription.Patients should use the antibiotics for the entiretime prescribed, should not end treatmentearly, and should not save antibiotics.

Viruses do not have the cell walls and cellmembranes of bacteria, nor do they have meta-bolic or protein synthesis machinery. Virusesare not susceptible to antibiotics. Viruses needhuman cells to reproduce and decode their ge-netic material. Some antiviral drugs interferewith this process by acting as nucleic acid ana-logues, substances that mimic the correct DNAor RNA bases. These analogues are used tomanufacture the viral genetic material but, infact, do not function as the normal DNA or RNAbases. Viruses are not replicated correctly andare eventually reduced in number or eliminated.Other antiviral drugs interfere with the assem-bly of new virus particles inside cells or interferewith the attachment of viruses to host cells andthus prevent infection before disease begins.

Antifungal drugs target fungal walls andmembranes but can affect human cells as well,leading to serious toxic side effects. Topicalagents are effective for skin infections, such asinfections of nails or ringworm, and pose feweradverse effects. A systemic infection, however,requires systemic treatment, which entails therisk of serious side effects. Systemic treatmentrequires careful dosing and monitoring for sideeffects.

Protozoa are treated with drugs that interferewith protein synthesis and metabolism. Certainantibiotics may be used to treat protozoal infec-tions.

Helminths are susceptible to drugs that par-alyze their muscles or interfere with their car-bohydrate metabolism.

Although effective treatments have been dis-covered and used for many important infections,certain problems remain. One complication isthat resistant microorganisms can evolve, ren-dering existing treatments useless. Another diffi-culty is that some treatments are accompaniedby unacceptable toxic side effects or allergies. Forthese reasons, preventive measures are the bestchoice for long-term control of certain diseases.


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Population Vaccines

All young children Measles, mumps, and rubella

Diphtheria-tetanus toxoid and pertussis

Poliomyelitis

Haemophilus influenzae type B

Hepatitis B

Varicella

Previously unvaccinated or partially Hepatitis Bvaccinated adolescents

Varicella

Measles, mumps, and rubella

Tetanus-diphtheria toxoid

All adults Tetanus-diphtheria toxoid

All adults aged >65 years Influenza

Pneumococcal

Table 3–3 Universally Recommended Vaccinations

VaccinationVaccination is the presentation of antigens froma microorganism to provoke an immune re-sponse in order to prevent future infection bythat microorganism. Natural immunity occurswhen a person is infected by or exposed to a mi-croorganism. The individual’s immune systemresponds and produces memory cells that areprimed to protect the person from future en-counters with that microorganism. The initialinfection can cause a great deal of discomfortand major complications, including death.Preventive vaccination is recommended andpracticed. Vaccines may contain dead bacteria,extracted antigens, inactivated toxins, virusparticles, or genetically engineered proteins.

Because they are designed to prevent disease,vaccines have been used to help eliminate certaindiseases. Smallpox was eliminated by thoroughvaccination of susceptible populations. Polio willbe the next major infectious disease to be elimi-

nated through vaccination. Administration of aseries of vaccines in infancy and childhood hascontrolled many serious childhood diseases.Diphtheria, whooping cough, measles, and othercontagious and potentially serious infections arenow much less common. Vaccines can protecthealth-care workers from hepatitis B and otherpathogens. Table 3–3 describes the recommenda-tions of the Centers for Disease Control andPrevention for vaccinations for various popula-tions. Further details are discussed in Chapter 2.

Emerging Infectious Diseases

Infections still account for a significant amountof morbidity and mortality in the United States.Recent data from the Centers for DiseaseControl and Prevention show that infectiousdisease mortality has increased significantly be-

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tween 1980 and 1996 from about 42/100,000to about 66/100,000. Moreover, certain infec-tions that were once considered under controlhave re-emerged as important health threats.

The factors behind the re-emergence of tu-berculosis are described in Chapter 9. One ofthe factors is increased antibiotic resistance ofthe tuberculosis pathogen. Antibiotic resistanceplays an important role in the increased inci-dence of other bacterial infections. In additionto antibiotic resistance, changes in climate, ur-banization, increased crowding, increased inci-dence of chronic disease, fast world travel, anddisruption of social and governmental structureare responsible for the emergence of newpathology or re-emergence of old infectious dis-eases.

Climate changes can alter the breedingranges of arthropod vectors like mosquitoes andflies. Malaria, Dengue fever, yellow fever—allmosquito-borne infections—show sensitivity toclimate. Even in areas where malaria is en-demic, it occurs with less frequency in higherand cooler elevations. In 1987, when the tem-perature averaged 1° higher than the previousyear in Rwanda, the incidence of malaria in-creased 337% because higher, cooler, drierareas became favorable for the malarial mosqui-toes.

After increased urbanization brought hu-mans and deer in close proximity, Lyme diseasebecame prominent in northeastern UnitedStates in recent years. Much of that area wascleared for farms, driving out deer and theirpredators. Forest and deer have reclaimed thefarmland, but the predators have not returned.As urban centers grow, the human and deerpopulations inevitably come in contact withmore frequency, giving the Lyme disease ticksample opportunity to attach to humans andtheir pets.

The population of the world exceeds 6 billionpeople. Most of the growth has occurred and

will continue to occur in the most densely pop-ulated and poorest cities. In the next few years,the World Health Organization projects that ofthe top 15 or so most populous cities, onlyTokyo will be in a so-called industrialized na-tion. Crowding, chronic disease, malnutrition,and lack of medical resources will become criti-cal. Infectious diseases thrive on these condi-tions.

Scientists must continue basic research intothe cause, transmission, prevention, and treat-ment of infectious diseases for future genera-tions.

C H A P T E R S U M M A R Y

Infectious diseases remain a significant cause of mor-bidity and mortality in the United States andthroughout the world. The diseases are caused by a

variety of pathogenic microorganisms, viruses, helminths,and arthropod vectors. The characteristics of each are im-portant to understand to implement effective preventionand treatment. As human populations grow, pathogenswill evolve, and infectious diseases likely will remain athreat to human life.

R E S O U R C E S

Biddle, W. 2002. A Field Guide to the Germs, 2nd ed.

Madigan, M.T., Martinko, J., & Parker, J. 2003. BrockBiology of Microorganisms, 10th ed.

American Society for Microbiology: www.asm.org

Centers for Disease Control and Prevention: www.cdc.gov


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Cases for Critical Thinking

1. Based upon what you learned about transmission and con-trol of infectious diseases, compare how one would ap-proach the control of influenza with the control of malaria.Explain which methods would be useful for each disease.

2. Explain why antibiotics are ineffective against viral infec-tions. What problems can arise when viral infections aretreated with antibiotics?

3. Explain why vaccination is a particularly effective methodfor controlling infectious disease.

Interactive Activities

1. A strain of cholera has appeared in unusual amounts insouth Asia, South America, and even in North America. Theoccurrence of this cholera strain can be described as_______________.

a. endemic c. epidemicb. pandemic d. outbreak

2. Viruses are different from bacteria in that viruses_______________.

a. cannot grow on their ownb. have have cell membranesc. have genetic materiald. are single-celled organisms

3. Malaria is caused by _______________.

a. a mosquito c. Trypanosomab. Plasmodium d. varicella virus

4. Penicillin disrupts bacterial _______________.

a. ribosomes c. cell wallsb. cell membranes d. DNA synthesis

5. Which of these is not a contagious disease?

a. influenza c. measlesb. strep throat d. malaria

6. The gram stain is used to differentiate types of_______________.

a. bacteria c. protozoab. viruses d fungi

7. _______________ are long, whiplike appendages used forswimming.

a. cilia c. myceliab. endospores d. flagella

8. The protein coat of a virus is its _______________.

a. nucleus c. coreb. capsid d. mycelia

9. Chitin is unique to _______________ cell walls.

a. bacterial c. fungalb. protozoal d. viral

10. Which is a fungal infection?

a. measles c. schistosomiasisb. rabies d. ringworm

Multiple Choice


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True or False

_______ 1. Most microorganisms cause disease.

_______ 2. Viruses are not considered living organisms because they do not independentlygrow, metabolize, or reproduce.

_______ 3. When an infectious disease is transmitted directly from an infected human to a sus-ceptible human, it is known as a horizontal transmission.

_______ 4. One of the factors behind the re-emergence of tuberculosis is increased antibioticresistance of the tuberculosis pathogen.

_______ 5. Ascaris is a large flatworm that infects the blood vessels.

_______ 6. Food, water, and insects can all transmit infectious diseases.

_______ 7. Amoeboids move by using pseudopodia.

_______ 8. Pinworms and tapeworms infest human intestines.

_______ 9. Antibiotics are effective treatments for helminth infestations.

_______ 10. Fungi normally do not infect healthy tissues.

Fill-Ins

1. Infectious diseases are those diseases caused by _______________ _______________.

2. _______________ is the presentation of antigens from a microorganism to provoke an immuneresponse and thus prevent future infection by that microorganism.

3. Animals with jointed legs and a hard exoskeleton are called _______________.

4. _______________ is the study of the transmission, occurrence, distribution, and control ofdisease.

5. Health-care workers must follow _______________ _______________ when handling patients.

6. Insects such as mosquitoes serve as _______________ of infectious disease.

7. A source of pathogens in nature is called a _______________.

8. Lyme disease is carried to humans by a species of _______________.

9. The number of diseases occurring in a specific time frame is known as its _______________.

10. The blood fluke _______________ causes disease in south Asia, Africa, and South America.


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MedMediaWrap-Up

Activity CD-ROM

Check out the CD-ROM in the backof this book. You will find games,exercises, puzzles, and videos tohelp enhance your understandingof this chapter.

www.prenhall.com/mulvihill

Remember to visit this website forextra study practice, includingexercises, Internet links, newsupdates, and an audio glossary.


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