Introduction

Many parasites exploit their host in order to accomplish within-host reproduction and allow transmission to new hosts. However, an extreme exploitation strategy may incur a cost since it might decrease the life expectancy of the host and, as a consequence, the chances of the parasite being transmitted. In this respect, virulence (i.e., the deleterious effect induced by the parasite) can be considered a byproduct of the parasite's host-exploitation strategy. Such a trade-off leads to the conclusion that parasites should evolve toward intermediate levels of virulence. This idea has been formalized by several authors (Anderson and May 1979; Ewald 1983; Van Baalen and Sabelis 1995a; Frank 1996c), who found that an evolutionarily stable level of virulence depends on several life-history parameters for both the host and parasite, as well as some constraints (such as the classic trade-off between transmission ability and virulence).

Moreover, it has been shown that multiple infections (i.e., the ability of the parasite to infect an already infected host) increase within-host competition and select for higher levels of virulence (Eshel 1977; Bremermann and Pickering 1983; Frank 1992a, 1994b, 1996c; Nowak and May 1994; May and Nowak 1995; Van Baalen and Sabelis 1995a). Several models have been developed around this idea (see Box 5.1), but we believe the kin-selection model proposed by Frank remains the simplest way to address this question (but see Box 11.1). Frank formalized the idea that there is a strong analogy between the evolution of altruism and that of parasite virulence.