Nonhuman primates contribute substantially to emerging infectious diseases in humans. All vector-borne parasites have a higher risk of becoming emerging infectious disease than parasites transmitted through other means have. The ecological context is of paramount importance in understanding under what conditions any given parasite may potentially be spread among species. In many cases among primate species, the barrier preventing an infectious agent in a host species from cross-infecting a novel species may not be due as much to biological differences as to ecological differences that prevent two primate species from coming into contact. This chapter focuses on wild primate diseases that cross over into human populations. Parasite amplification in a host population is necessary for the parasite to achieve sufficient densities to be effectively transmitted. Reservoir hosts are amplification hosts, but amplification may also occur in spillover infections. Spillover infections may result in high levels of virulence, and secondary spillovers are also possible.