As is the case with almost all free-living organisms, parasites belonging to a particular species rarely occur alone, although this can happen at the scale of host individual. In most cases, parasites belonging to different species co-occur forming a community. Analogously with assemblages of conspecific parasites, spatial distribution of parasite communities is fragmented among host individuals, among host species within a location and among locations. It is logical thus that the hierarchical terminology aimed at distinguishing between parasite communities at different scales will be parallel to that adopted for parasite populations. Consequently, Combes (2001) suggested that we should distinguish infracommunity (an assemblage of parasites of all species infesting an individual host), xenocommunity (= component community; an assemblage of parasites of all species infesting a host population) and community (= compound community; an assemblage of parasites of all species infesting a host community). I use this terminology throughout this chapter, albeit with some modifications.

The scale of consideration is extremely important when parasite communities are studied. In particular, this stems from the fact that there are at least two principal differences between infracommunities, on the one hand, and parasite assemblages at higher scales, on the other hand. First, this is related to temporal persistence of an assemblage. Infracommunities are short-living by definition, whereas xenocommunities and communities persist much longer. Second, this is related to possible interactions among parasite species. Occurrence of the interactions among heterospecific parasites is not necessarily the rule.