There is a troubling outbreak of yellow fever currently affecting a number of South American countries, including Brazil and Paraguay. Yellow fever is a multi-host infection that can be transmitted between monkeys and humans. It is almost always fatal for species in the genera Alouatta and Ateles (howler and spider monkeys respectively). The Wildlife Conservation Society (WCS) has taken note of the real conservation concerns that arise for small populations of these monkeys in the context of the continuing yellow fever outbreak. In a world of increasingly fragmented local wildlife populations, where human-wildlife contact increases from human intrusion of forests and other ecosystems, control multi-host epidemics is likely to become an integral part of the conservation ecologist's portfolio.