Introduction

Highly pathogenic epidemic disease agents, like the rinderpest and myxomatosis viruses, have obvious and often dramatic consequences for the dynamics and evolution of their host populations (Osterhaus and Vedder 1988; Roelke-Parker et al. 1996; Vogel and Heyne 1996; Hudson 1997; Hochachka and Dhondt 2000). In contrast, the effects of endemic diseases are often more subtle and, until recently, had largely been overlooked (Grenfell and Dobson 1995; Hudson 1997; Hudson et al. 2002). The mathematical models of Anderson and May in the late 1970s (Anderson and May 1978, 1979, 1982a; May and Anderson 1978, 1979) were the first to highlight the potential of endemic parasites and pathogens to regulate host populations, and more recent theoretical studies have also implicated parasites as potentially important driving forces in the evolution of their hosts. Their impact is believed to extend to the evolution of secondary sexual characters (Hamilton and Zuk 1982; Read 1987, 1988; McLennan and Brooks 1991; Hamilton and Poulin 1997) and optimal life-history strategies (Michalakis and Hochberg 1994; Sheldon and Verhulst 1996; Richner 1998); the manipulation of host behaviour (Moore 1984; Poulin 1994; Moore and Gotelli 1996); the maintenance of genetic diversity and even to the evolution of sex (Van Valen 1973; Hamilton 1980; Hamilton et al. 1990; Moritz et al. 1991; Howard and Lively 1994).