AIDS: Studies of HIV nonprogressors and rapid progressors reveal three components of the host-virus interaction that determine a person's ability to control virus replication: mutations in the virus itself, innate genetic host factors, and host immune responses to HIV. It remains unclear why some people have effective immune responses to HIV and others do not. Research shows that nonprogressors have persistent, vigorous, virus-inhibiting cytotoxic T lymphocytes (CTLs) whose response is broad and adaptable, whereas rapid progressors have a narrowly directed CTL response that is unable to adapt to changes in the virus. Increased attention is now focusing on the beginning of primary HIV infection to help determine immunologic dysfunction throughout the course of the disease. A hypothesis suggests that aggressive antiretroviral therapy begun during acute HIV infection may allow the generation of an immune response that otherwise would be absent and allow the host defense system to effectively control viral replication. Several case reports demonstrating different types of immune response to HIV are presented.