THE LEISHMANIASES AS MODEL ZOONOSES

Of the 374 species of parasite recorded as naturally infecting Homo sa piens, all but some 40 are zoonotic. A classification of human parasit es 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 num ber of host-specific forms. The classification is illustrated by vario us species of Leishmania, and these are used to illustrate various hos t-parasite systems. The 'Wells effect' is proposed, to describe the pr otection of host populations by their parasites.