All six great apes, gorillas Gorilla gorilla and G. beringei, chimpanzees Pan troglodytes and P. paniscus, and orang-utans Pongo pygmaeus and P. abelii, are categorized as Endangered on the 2000 IUCN Red List and face many threats to their continued existence in the wild. These threats include loss of habitat to settlement, logging and agriculture, illegal hunting for bushmeat and traditional medicine, the live ape trade, civil unrest and infectious diseases. The great apes are highly susceptible to many human diseases, some of which can be fatal while others can cause marked morbidity. There is increasing evidence that diseases can be transmitted from humans to free-living habituated apes, sometimes with serious consequences. If protective measures are not improved, ape populations that are frequently in close contact with people will eventually be affected by the inadvertent transmission of human diseases. This paper describes the risks, sources and circumstances of infectious disease transmission from humans to great apes during and consequent upon habituation for tourism and research. A major problem is that the regulations that protect habituated apes from the transmission of disease from people are often poorly enforced. Suggestions are made for improving the enforcement of existing regulations governing ape-based tourism, and for minimizing the risk of disease transmission between humans, both local people and international visitors, and the great apes.