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486 CHAPTER TWENTY Epidemiology

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1940 1960Year

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FIGURE 20.8 Crude Death Rate for Infectious Diseases, UnitedStates, Per 100,000 Population per Year

deaths caused by Haemophilus influenzae. Corynebacterium diph-theriae, Clostridium tetani, and Bordetella pertussis. Meanwhile,recognizing and controlling the source of diseases such as malaria,plague, and cholera have been effective in limiting their spread.

Emerging DiseasesJust as humans have been successful in reducing and eliminating cer-tain diseases. microorganisms are equally adept at taking advantageof new opportunities in which to thrive and multiply. As human life-styles change due to advancing technologies, increasing populations,and shifting social behaviors, new diseases emerge while those thathave been controlled in the past sometimes make a comeback.

Diseases that have increased in incidence in the past twodecades are referred to as emerging diseases. These include newor newly recognized diseases such as SARS as well as familiarones such as malaria that are reemerging after years of decline (seefigure 1.4). Some of the factors that contribute to the emergenceand reemergence of diseases include:

_ Microbial evolution. The emergence of some diseases is dueto the natural evolution of microorganisms. For example, anew serotype of Vibrio cholerae, designated 0139. appears tobe nearly identical to the strain that most commonly causescholera epidemics, V. cholerae 0 I, except that it has gainedthe ability to produce a capsule. The consequence of the newserotype is that even people who have immunity against theearlier strain are susceptible to the new one. Resistance to theetIects of antimicrobial drugs is contributing to the reemer-gence of many diseases, including malaria .• V. choleree, p, 624

• malaria, p. 725

- Complacency and the breakdown of public health infra-structure. As infectious diseases are controlled and thereforeof lessening concern, complacency can develop, paving theway for the resurgence of a disease. The preliminary success

FIGURE 20.9 A Case of Smallpox, a Disease ThatEradicated

of the plan to eliminate tuberculosis in the Unitedthe year 2000 resulted in less public attention being,disease. News reports, education, and research modiverted to more common diseases. Simultaneouslcrisis developed and funding for some social weltwas curtailed, resulting in an increase in the nurnbeat risk of developing active tuberculosis due to poorliving conditions. Consequently, tuberculosis reernincreasing threat. Fortunately, increased public hsures, including direct observation of drug therapybrought the disease back under at least temporaryfigure 24.16) .• tuberculosis, p. 593

- Changes in human behavior. Changes in society'~jcan inadvertently create opportunities for microor~;spread and flourish. For example, day care centers,:pered infants mingle, oblivious to sanitation and ha relatively new component of American society. HIreasons, the centers can be hotbeds of contagious':Many young children have not yet acquired i,common communicable diseases. As a consequencesuch as colds and diarrhea are readily transmittedsusceptible population. This is particularly true wi ,pathogens such as Giardia and Shigella that have IH"

tious dose, because infants often explore throug~:)touch and are thus likely to ingest fecal organismi;p. 638 • Shigella, p. 625

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