FORMULATION OF THE SIMULATION MODEL FOR HOMOSEXUAL MEN Transmissions of HIV have occurred after exposure to people in the pre-antibody period

Transmissions of HIV have occurred after exposure to people in the pre-antibody period (Peterman et al., 1985) . The latent period (i .e ., from infection to infectiousness) for HIV infection is short enough to be ignored in a model. The infectious period is unknown, but the virus seems to persist in the host indefinitely since it can be isolated from the blood for many years after the infection (Curran et al ., 1988) . No evidence to the contrary exists ; therefore HIV infectivity is assumed to continue for life . Since individuals are either susceptible to HIV infection or infectious, HN is an SI (susceptible-infected) disease (Hethcote, 1978, 1989c) . In contrast, gonorrhea is an 5IS disease since people can recover and return to the susceptible class .