SHORT-TERM EVOLUTION OF VIRULENCE
The evolution of virulence is a widely misunderstood process. Conventional wisdom has it that parasites should always be evolving toward reduced virulence. The reason- ing assumes, correctly, that the longer the host lives, the
longer the parasites can live and the longer they can disperse off- spring to new hosts. Any damage to the host on which they depend will ultimately damage all dependent parasites, and the most success- ful parasites should be those that help the host in some way. The expected evolutionary sequence starts with a virulent parasite that becomes steadily more benign until finally it may become an impor- tant aid to the host’s survival.
There are several things wrong with this seemingly reasonable argument. For example, it ignores a pathogen’s ultimate requirement of dispersing offspring to new hosts. This dispersal, as noted in the previous chapter, frequently makes use of host defenses, such as coughing and sneezing, that are activated only as a result of apprecia- ble virulence. A rhinovirus that does not stimulate the host to defend itself with abundant secretion of mucus and sneezing is unlikely to reach new hosts.
