We analyze the spread of sexually transmitted infections in an environment where individuals search for a sexual partner and, when found, cannot verify whether his partner is infected. Decisions are based on a variety of factors including the proportion infected, the likelihood of safe sex, the rate of detection and treatment, the cost of infection, and the length of search. The model demonstrates how directed search induces a separating equilibrium and, as a result, supports empirical evidence demonstrating the importance of sero-sorting. Furthermore, the model reinforces arguments that decreasing the costs of infection increases the infection rate. The model is calibrated, and the policy implications are analyzed within the context of men who have sex with men. Copyright © 2012 John Wiley & Sons, Ltd.