Since the beginning of the pandemic, three main transmission routes for HIV have been identified:

Sexual route. The majority of HIV infections are acquired through unprotected sexual relations. Sexual transmission can occur when infected sexual secretions of one partner come into contact with the genital, oral, or rectal mucous membranes of another.
Blood or blood product route. This transmission route can account for infections in intravenous drug users, hemophiliacs and recipients of blood transfusions (though most transfusions are checked for HIV in the developed world) and blood products. It is also of concern for persons receiving medical care in regions where there is prevalent substandard hygiene in the use of injection equipment, such as the reuse of needles in Third World countries. HIV can also be spread through the sharing of needles. Health care workers such as nurses, laboratory workers, and doctors, have also been infected, although this occurs more rarely. People who give and receive tattoos, piercings, and scarification procedures can also be at risk of infection.
Mother-to-child transmission (MTCT). The transmission of the virus from the mother to the child can occur in utero during the last weeks of pregnancy and at childbirth. In the absence of treatment, the transmission rate between the mother and child is 25%.However, where drug treatment and Cesarian section are available, this can be reduced to 1%.Breast feeding also presents a risk of infection for the baby.
HIV-2 is transmitted much less frequently by the MTCT and sexual route than HIV-1.

HIV has been found at low concentrations in the saliva, tears and urine of infected individuals, but there are no recorded cases of infection by these secretions and the potential risk of transmission is negligible.The use of physical barriers such as the latex condom is widely advocated to reduce the sexual transmission of HIV. Spermicide, when used alone or with vaginal contraceptives like a diaphragm, actually increases the male to female transmission rate due to inflammation of the vagina; it should not be considered a barrier to infection.Trials, in which uncircumcised men were randomly assigned to be medically circumcised in sterile conditions and given counseling and other men were not circumcised, have been conducted in South Africa,Kenya[25] and Uganda showing reductions in HIV transmission for heterosexual sex of 60%, 53%, and 48% respectively. As a result, a panel of experts convened by WHO and the UNAIDS Secretariat has "recommended that male circumcision now be recognized as an additional important intervention to reduce the risk of heterosexually acquired HIV infection in men.Research is clarifying whether there is a historical relationship between rates of male circumcision and rates of HIV in differing social and cultural contexts. Critics point out that any correlation between circumcision and HIV is likely to come from cultural factors (which govern not only whether someone is circumcised, but also their sexual practices and beliefs). South African medical experts are concerned that the repeated use of unsterilized blades in the ritual (not medical) circumcision of adolescent boys may be spreading HIV.

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AIDS stands for acquired immunodeficiency syndrome. It is the most advanced stages of infection with the human immunodeficiency virus (HIV).