The past 15 years of research have made it clear that evolutionary processes do not mandate benign associations between parasite and host when short-term forces of survival and reproduction push in the opposite direction. Just as with predators and prey, natural selection can lead to an unending series of population expansions and crashes. The conditions that engender plagues always end sooner or later. This always, always means that plague pathogens will not survive in the long term unless some of them manage subsequently to lose that virulence and revert to the habits of their low-profile relatives. This conclusion roams beyond the realm of evidence.
Plagues involve the presence of virulent pathogens in a large number of hosts.
The end of a plague may or may not be associated with selection for lowered virulence. Many plagues burn themselves out because enough hosts die so that their population density decreases and the pathogen can no longer be transmitted effectively. If the selective pressures on the competing strains of pathogens favor the more vicious variants, however, the pathogens may maintain high virulence. A low profile may result from a low rate of infection rather than a loss of virulence. Wills employs colorful metaphors to make some complex evolutionary ideas comprehensible, but his use of analogy also leads him astray.