The predominant mode of HIV-1 transmission globally is from sexual practices. The risk of HIV-1 transmission by sexual means is a function of infectivity, susceptibility and mode of transmission (type of sexual practice). In addition, transmission may be significantly increased or decreased by factors relating to the HIV-1 per se, sexual behaviour, other sexually transmissible infections (STIs), antiretroviral therapy (ART), spermicidal microbicides and HIV-1 vaccines, the host immune system, genital anatomy and nutritional deficiencies. Current research into the factors affecting sexual transmission of HIV-1 appears to benefit developed nations more than developing nations because of structured public health systems and the capacity to translate research findings into prevention strategies. A redistribution of global aid funding would do much to alleviate the pandemic in developed countries.