  In the pool of people seeking partners, a uniformly greater preference for
abstinence increases the prevalence of infection and worsens everyone's
welfare. In contrast, prevention and treatment reduce prevalence and improve
payoffs. The results are driven by adverse selection: people who prefer more
partners are likelier disease carriers. A given decrease in the number of
matches is a smaller proportional reduction for people with many partners, thus
increases the fraction of infected in the pool. The greater disease risk
further decreases partner-seeking and payoffs.
