Mountain gorilla Gorilla beringei beringei tracking tourism generates important revenue for conservation efforts but brings with it the threat of disease transmission into the gorilla population. This study quantifies for the first time aspects of encounters between gorillas and tourists at Bwindi Impenetrable National Park that are likely to contribute to the risk of disease transmission. These include how close tourists get to gorillas, how close encounters are initiated, how long they last, and the age class of gorillas involved. Tourists got significantly closer to gorillas than the park rules allow (a mean of 2.76 m, compared to the rule of 7 m), and remained close for long periods. Contacts with the gorillas most vulnerable to disease, the juveniles, were closer but of shorter duration than those with adults. Contacts initiated by gorillas were closer but shorter than those initiated by tourists. Taken together these results demonstrate that the present rules are failing, and that the risk of disease transmission may be greater than previously believed.