These monkeys host arboviruses as well as the primate malaria species Plasmodium knowlesi which can also infect humans.  Key determinants of whether macaques and people share parasites include proximity of human habitation to the forest, the location and feeding preferences of mosquito vectors, and relative susceptibility of the host species.  Zoonotic reservoirs like these can play a major role in the evolution and persistence of parasites. And for species of conservation concern, parasites can have an important effect on likelihood of extinction.