This paper analyzes the dynamic interplay between individuals beliefs over transmission risks and their decisions on whether to engage in unprotected sexual behavior. The infectious disease is (possibly) asymptomatic, which makes the inference problem non-trivial. In general, the dynamics of beliefs can be non-monotone in time, even without shifts in actions. Both monogamous and concurrent relationships are considered. It is shown that if unprotected sex is voluntary, then a utilitarian social planner would prefer more unprotected sex than the level chosen in equilibrium. Furthermore, it is shown that altruism towards the partner will increase the amount of voluntary unprotected sex and thus increase the amount of infection.