Core groups appear to play an important role in the heterosexual spread of HIV both in industrialized and in developing countries. Condoms are an effective prevention measure and the well organized condom campaign in Thailand has been shown to have a significant impact on the incidence of sexually transmitted diseases and HIV. Sexually transmitted diseases facilitate the sexual transmission of HIV and preliminary evidence suggests that improved sexually transmitted disease case management can be successful in lowering the HIV incidence in developing countries. There is some evidence that certain HIV-1 subtypes are associated with a higher risk of heterosexual transmission.