Secondly, host switches would be favored if the new parasite, as exemplified by HIV, directly attacks the host's immune system and compromises it, or if the new host is immunocompromised by some other means. Diminished levels of immune competence can occur for several reasons, including ones likely to have been in operation throughout animal evolution. One possible means is that the host's indigenous parasites might use immunosuppression to favor initiation and persistence of their own long-term infections ( Table 6 ) and thereby facilitate colonization of that host by other parasites (Krasnov et al., 2005) . An intriguing possibility is that the successful colonization of a host species by one or more immunosuppressive parasites might then favor colonization by opportunist parasites, resulting in an unusually diverse parasite fauna supported by that host. The large number of species of larval digenetic trematodes known to be supported by some snail species (Loker et al., 1981; Lafferty and Kuris, 2009b) might exemplify this possibility.