The rapid scientific advances made over the past several decades led some people to speculate at one point that the war against infectious diseases, particularly those caused by bacteria, had
been won. Microorganisms have occupied this planet far longer than have humans, however, evolving to occupy every habitat having the potential for life, including the human body. Perhaps it should be no surprise, then, that new and previously unrecognized pathogens are emerging, and that some of those we had previously controlled are now making a comeback.
Humans have been enormously successful in developing the means to eliminate or reduce the occurrence of certain diseases through such efforts as improved sanitation, reservoir and vector control, vaccination, and antibiotic treatment. One disease, smallpox, has been globally eradicated, eliminating the natural occurrence of a disease that had a 25% mortality rate, and left many of those who survived permanently disfigured (figure 20.9). The World Health Organization hopes to eradicate polio by the end of the year 2005. Dracunculiasis and measles are also targeted for elimination. Whether these efforts will be successful remains to be seen. Political and social upheaval, complacency, and lack of financial support can result in a resurgence of diseases unless the pathogens are completely eliminated. ■ polio, p. 677 ■ measles, p. 549
In the United States, many diseases that were once common and claimed many lives are now relatively rare. Successful
498 Chapter 20 Epidemiology vaccination programs have led to dramatic decreases in the number of deaths caused by Haemophilus influenzae, Coryne-bacterium diphtheriae, 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.
Just as humans have been successful in reducing and eliminating certain diseases, microorganisms are equally adept at taking advantage of new opportunities in which to thrive and multiply. As human lifestyles change due to advancing technologies, increasing populations, and shifting social behaviors, new diseases emerge while those that have been controlled in the past sometimes make a comeback.
Diseases that have increased in incidence in the past two decades are referred to as emerging diseases. These include new or newly recognized diseases such as Ebola as well as familiar ones such as malaria that are reemerging after years of decline. Figure 20.10 shows the emergence of diseases around the globe. Some of the factors that contribute to the emergence and reemergence of diseases include:
■ Microbial evolution. The emergence of some diseases is due to the natural evolution of microorganisms. For example, a new serotype of Vibrio cholerae, designated O139, appears to be nearly identical to the strain that most commonly causes cholera epidemics, V. cholerae O1, except that it has gained the ability to produce a capsule. The consequence of the new serotype is that even people who have immunity against the earlier strain are susceptible to the new one. Resistance to the effects of antimicrobial drugs is contributing to the reemergence of many diseases, including malaria. ■ V. cholerae, p. 611 ■ malaria, p. 731
■ Complacency and the breakdown of public health infrastructure. As infectious diseases are controlled and therefore of lessening concern, complacency can develop, paving the way for the resurgence of a disease. The preliminary success of the plan to eliminate tuberculosis in the United States by the year 2000 resulted in less public attention being paid to the disease. News reports, education, and research money were all diverted to more common diseases. Simultaneously, the AIDS crisis developed and funding for some social welfare programs was curtailed, resulting in an increase in the number of people at risk of developing active tuberculosis due to poor health and living conditions. Consequently, tuberculosis reemerged as an increasing threat. Fortunately, increased public health measures, including direct observation of drug therapy compliance, brought the disease back under at least temporary control (see figure 23.16). As another example, a decreased vigilance in ensuring childhood immunization resulted in the resurgence of measles in the late 1980s. ■ tuberculosis, p. 580 ■ measles, p. 549
■ Changes in human behavior. Changes in society's norms and behavior can inadvertently create opportunities for microorganisms to spread and flourish. For example, day care centers, where diapered infants mingle, oblivious to sanitation and hygiene, are a relatively new component of American society. For obvious reasons, the centers can be hotbeds of contagious diseases. Many young children have not yet acquired immunity to common
communicable diseases. As a consequence, illnesses such as colds and diarrhea are readily transmitted among this susceptible population. This is particularly true with intestinal pathogens such as Giardia and Shigella that have a low infectious dose, because infants often explore through taste and touch and are thus likely to ingest fecal organisms. ■ Giardia, p. 625
■ Advances in technology. Technology can make life easier but can inadvertently create new habitats for microorganisms. For example, the advent of contact lenses to correct vision gave microorganisms the opportunity to grow in a new location, the lenses and storage solutions of users who did not employ proper disinfection techniques. In turn, this resulted in new types of eye infections.
■ Population expansion. The increase in world population, and the subsequently denser living conditions, create situations in which diseases can be more readily transmitted. In areas where the population has expanded outward, people are coming in contact with previously unknown reservoirs of disease such as that of the Ebola virus. The reservoir of Ebola still has not been determined.
■ Development. Dams, which provide important sources of power necessary for economic development, have inadvertently extended the range of certain diseases. For example, transmission of the disease schistosomiasis relies on the presence of an aquatic snail that serves as a host for the Schistosoma parasite. Construction of dams such as the Aswan dam on the Nile River has increased the habitat for the snail, thus extending the distribution of the disease. ■ schistosomiasis, p. 388
■ Mass distribution and importation of food. Foodborne illness has always existed, but the ease with which we can now transport items worldwide can create new problems. Widespread distribution of foods contaminated with pathogens can result in a similarly broad outbreak of disease. For example, contaminated raspberries grown in Guatemala were linked to a 1996 multistate outbreak of diarrheal disease in the United States, affecting more than 900 people, caused by the intestinal parasite Cyclospora. ■ Cyclospora, p. 627
■ War and civil unrest. Wars and civil unrest can disrupt the infrastructure on which disease prevention relies. Refugee camps that crowd people into substandard living quarters lacking toilet facilities and safe drinking water are hotbeds of infectious disease. Epidemics of cholera, dysentery, and other infectious diseases are common in these situations. Unfortunately, war also disrupts disease eradication efforts. ■ cholera, p. 611
■ Climate changes. Changes in temperature and rainfall may affect the incidence of certain diseases. For example, warm temperatures favor the reproduction and survival
of some arthropods, which in turn can serve as vectors for diseases such as malaria. The heavy rainfall and flooding that resulted in a surge of cholera cases in Africa may have been due to the effects of El Niño. ■ malaria, p. 731 ■ cholera, p. 611
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