Although the transmission of Pneumocystis carinii between mammals of the same species has been convincingly demonstrated in the laboratory, it is unproven whether a similar clinical model exists for human disease. In addition, it is debated whether PCP in humans develops solely from reactivation or of a latent infection acquired early in childhood or also from acquisition of a new infection due to a recent exposure. The proposed research plan will determine whether P. carinii and immune responses to P. carinii can be detected in household and hospital contacts living with and caring for HIV-infected patients with PCP and, if P. carinii is detected, whether the P. carinii genetic sequence from a contact matches that from the patient with PCP. This information will be used to develop future studies to examine whether PCP can result from acquisition of a new infection. Since PCP remains a leading cause of the moribiity and mortality in HIV-infected and other immunocompromised persons, insights into its epidemiology and transmission may have profound implications for clinical management and disease prevention.