The Power of Plagues. Irwin W. Sherman

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The Power of Plagues - Irwin W. Sherman


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with other potential hosts, thereby reducing the possibilities for virus transmission by direct contact. Similarly, the development of diarrhea in a person with the disease cholera or Salmonella infection (which causes “food poisoning”) facilitates the dispersal of these intestinal microbes via fecally contaminated water and food, and in the absence of diarrhea parasite transmission would be reduced.

      AIDS is a consequence of an increase in the virulence of HIV. The enhancement in HIV virulence is believed to have resulted from accelerated transmission rates due to changes in human sexual behavior: the increased numbers of sexual partners was so effective in spreading the virus that human survival became less important than survival of the parasite. As the various kinds of plagues are considered in greater detail in subsequent chapters, recognition of the evolutionary basis for virulence may suggest strategies for public health programs. Clean water may thus favor a reduction in the virulence of waterborne intestinal parasites (such as cholera), and clean needle exchange and condom use would both reduce transmission and lessen HIV virulence. But some contend that this indirect mechanism may be too weak and too slow to reduce virulence substantially, and that a better approach could be direct selection by targeting the virulence factor itself. For example, immunization that produces immunity against the toxin produced by the diphtheria microbe also results in a decline in virulence. Future efforts will determine which strategy is the better means for effective “germ” control to improve the public’s health.

      1 *See: Cells and Their Structure in the Appendix

      2

      Plagues, the Price of Being Sedentary

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      Becoming Human, Becoming Parasitized

      Descent from the trees to the ground placed the australopithecines into a new environment, an ecological niche that was very different from the forest canopy. This freed them from some diseases but allowed for the acquisition of new ones. For example, in the treetops australopithecines would have been bitten by mosquitoes that carried parasites acquired from other animals living in the canopy, but at ground level they would be exposed to other airborne bloodsuckers such as ticks and flies, or they would come in contact with different food sources and contaminated water. Their teeth were small and underdeveloped, as in modern human beings, and the canines, highly developed in existing ape species, were small like ours. We can infer from their teeth that these australopithecines probably chewed fruits, seeds, pods, roots, and tubers. Since no stone tools have been found associated with the fossils, it is believed that A. afarensis did not make or use durable tools or understand the use of fire. They were opportunistic scavengers or vegetarians. The life span of an australopithecine has been estimated to have been between 18 and 23 years.

      Courtesy of Ken Mowbry, American Museum of Natural History


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