The factors influencing the transmission dynamics of HIV are discussed in the context of the community wide use of antiviral therapy. A mathematical framework is constructed to examine the impact of chemotherapy on transmission, based on a description of the progressive decline of CD4 counts in infected patients. Treatment which acts to increase the incubation period before the onset of serious immunodeficiency, but has no impact on the infectiousness of a patient, is obviously beneficial to the individual. However it can act to either reduce or increase net morbidity and mortality when used on a community wide basis. When treatment prolongs the incubation period and reduces infectiousness, community based chemotherapy is invariably beneficial to the individual and the community. Heterogeneity in infectiousness over the incubation period of AIDS, with peaks in early and late stages of infection, has an important influence on the effect of treatment on the net transmission within the community. In these circumstances community wide benefit is influenced by the timing of treatment within the incubation period of AIDS. The many heterogeneities arising in the transmission dynamics of HIV argue for careful analysis of the net impact of antiviral therapy on mortality at a community wide level as well as at the level of the individual patient.