Lessons from naked apes and their infections.

Human infections come from two main sources. Our 'family heirlooms' have co-evolved with the host as we diverged from the common ancestor of humans and chimpanzees, and these are often vertically transmitted. Our 'new acquisitions' come from cross-species infections, and these are typically horizontally transmitted. Compared with other apes, naked apes harbor a larger variety of pathogens, acquired from the domesticated and commensal non-primate species which share our habitat, as well as from exotic species. Thus we are nouveaux riches in our collection of infections or 'metagenome' and this is reviewed with particular reference to retroviruses. Nakedness poses a challenge to ectoparasites which is discussed in relation to the origin and evolution of human lice from those of the great apes. As humans have acquired infections horizontally from our closest living relatives, the chimpanzee and the gorilla, might we also have exchanged pathogens with other hominid species?