As the previous chapter discussed, patterns of disease had shifted markedly by the beginning of the 20th century. No longer did most people in Western nations die young of infectious or parasitic diseases (even though such diseases still ravaged poorer nations). Instead, most children survived childhood without major illnesses, and most adults died in old age of chronic disease. Yet infectious disease is on the rise once again.
The rising threat from infectious diseases first made medical news in 1981 when the first cases of what would become known as HIV/AIDS were identified. Since then, other new infectious diseases (such as Zika and Ebola virus diseases) have been identified, long-established diseases (such as cholera and streptococcus) have become deadlier, and previously harmless microorganisms (such as the virus that causes avian influenza or so-called bird flu) have caused important disease out- breaks.
The renewed dangers posed by infectious disease partly reflect basic principles of natural selection. Just as natural selection favors animals whose camouflaging coloration hides them from predators long enough to reproduce, natural selection favors microorganisms that can resist drug treatments. As doctors prescribed anti- biotics more widely, often under pressure from patients who feel cheated if they don’t receive a prescription each visit, the drugs killed all susceptible variants of disease-causing microorganisms while allowing variants resistant to the drugs to flourish. Similarly, drug-resistant tuberculosis is increasing in nations where HIV/ AIDS and poverty leave individuals both more susceptible to infection and less able to afford consistent, effective treatment. Meanwhile, the growing use of an- tibiotics in everything from cutting boards to kitty litter, chicken feed, and soaps also encourages the rise of drug-resistant bacteria.
Other forces also promoted the rise in infectious diseases. In the same way that population growth and the rise of cities once fostered the spread of infectious diseases in Europe, they are now causing new epidemics in rapidly growing cities in Africa, Asia, and Latin America. Meanwhile, older cultural traditions often erode among those who move to these cities, making health-endangering activities such as tobacco smoking and sexual experimentation more likely. At the same time, industrial plans and cities are growing into former forests and farmlands, bringing wild animals increasingly into contact with humans. As a result, microorganisms that previously had infected only animals now have the opportunity to infect humans as well