The leishmaniases as model zoonoses.

Of the 374 species of parasite recorded as naturally infecting Homo sapiens, all but some 40 are zoonotic. A classification of human parasites is proposed, based on the role of H. sapiens in the maintenance of suprapopulations. Categories range from 1, for which man is irrelevant to the persistence of the species, to 4, which includes the small number of host-specific forms. The classification is illustrated by various species of Leishmania, and these are used to illustrate various host-parasite systems. The 'Wells effect' is proposed, to describe the protection of host populations by their parasites.