HIV is present in the blood (including menstrual blood), semen and vaginal and rectal secretions of infected people, but can only be passed on to another person if those fluids get into his/her body. Although sophisticated laboratory techniques are able to isolate the virus from other body fluids of infected people, such as saliva, the level of virus in these fluids is far too low to be infectious.

Thus, the main ways in which HIV is transmitted are:

• Through unprotected anal or vaginal sex. HIV is unable to pass through good quality condoms, such as those bearing the CE mark, if they are used consistently and properly.
• It has been a subject of controversy for years as to whether HIV can be passed on during oral sex. It now does look as if a small proportion of infections – from 0.5% to 3% - get passed on this way. The risk is solely to the ‘receptive’ partner, i.e. the one ‘doing it’.
• Through blood-to-blood contact. This mainly happens through the sharing of injecting equipment amongst injecting drug users. In the past, before screening was introduced in the UK, this also occurred through blood transfusions or from infected blood products, such as the Factor VIII used to treat hæmophilia. Occasionally it happens through occupational accidents amongst healthcare workers, such as needlestick injuries. Fortunately, follow-up studies have shown that fewer than 1% of individuals who receive injuries with HIV-contaminated needles become infected
• ‘Vertically’: this means from mother-to-baby during the course of pregnancy, birth or breast-feeding. The average risk of transmission during pregnancy is in the region of 25% if the mother is HIV-positive, although it may be higher if she has a higher viral load (the amount of HIV in her blood, which indicates the rate at which the virus is reproducing in the body) or has developed AIDS. Breast-feeding may add another 10-15% of infections, and should be avoided in countries where good alternatives to the mother's milk are available.

Detailed discussion of the modes of transmission of HIV can be found in HIV transmission.