From the Black Death of the fourteenth century to the flu pandemic of 1918, disease has played a major role in human history. In the Anthropocene Period, we have unprecedented opportunity to control or even eradicate some of the great diseases of the past and present. At the same time, however, the strong selective pressures placed on pathogens by antibiotics pose a key challenge for 21st-century medicine.

Malaria, discussed in Case 4, provides an illustrative example. In any given year, malaria infects an estimated 500 million people, causing a million or more deaths. Infection is particularly severe in parts of Africa, where the direct and indirect effects of malaria account for almost half of all childhood mortality. Insecticide-soaked bed nets have proven highly effective in affected areas, reducing transmission by as much as 90%. Natural selection, however, has begun to chip away at success in controlling the disease: Mosquito strains have begun to emerge that are resistant to DDT and another family of insecticides called pyrethroids.

Quinine, a chemical extracted from the bark of the cinchona tree, was the first effective treatment for malaria, and remained in widespread use until the 1940s. Since that time, a small arsenal of antibiotics has been developed, including chloroquine, mefloquine, primaquine, and doxycycline. Their usefulness, however, has declined through time as resistance to each of these drugs has evolved in populations of Plasmodium, the infectious agent of malaria. More recently, artemisinins, antibiotics based on a traditional Chinese plant remedy, have proved effective in treatment. Again, however, resistant strains of malaria have been identified.

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Quick Check 5 The interplay of pathogens and antibiotics again brings up the problem of the Red Queen. How does the Red Queen hypothesis apply to these kinds of interaction?

The arms race between antibiotics and pathogens is not limited to malaria. It characterizes nearly the whole range of diseases caused by viruses, bacteria, protists, and fungi that infect humans. For example, tuberculosis, a lung infection spread by inhalation of the bacterium Mycobacterium tuberculosis, causes 1.8 million deaths annually, mostly in sub-Saharan Africa, where many people are also infected with HIV (Chapter 43). Antibiotic treatment is effective, but many who are infected do not or cannot comply with the strict regimen of daily treatment for 6 months, hastening the evolution of drug-resistant M. tuberculosis strains. This occurs because an incomplete course of antibiotics does not eradicate all the bacteria, selecting for those bacteria that are least susceptible to the antibiotic. In 2012, nearly half a million new cases of antibiotic-resistant tuberculosis were reported. More troubling, a small percentage of these infections have been characterized as “extremely drug resistant,” meaning that the pathogens resist both standard treatments and alternative antibiotics developed over the past 20 years (Fig. 49.23). In effect, extremely drug-resistant tuberculosis is untreatable.

Because bacteria can pass genes horizontally (Chapter 26), antibiotic resistance evolved in one species can jump to another. Thus, the bacterial incubators for drug resistance can arise anywhere and spread rapidly.

Clearly, despite the remarkable gains of the twentieth century, public health will remain a key issue in the decades ahead. Improved sanitation—clean drinking water and safe food—can do much to reduce transmission of infectious diseases, and the search continues for vaccines that can further reduce or even eliminate some historic scourges. But the ongoing evolution of drug resistance requires the continuing efforts of scientists to stay one step ahead of pathogens.