HIV is present in the blood (including menstrual blood), semen and vaginal fluids of infected people, but can only be passed on to another person if those fluids get into that person's 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.

The main ways in which HIV is transmitted are:

• Through unprotected anal or vaginal sex. HIV is unable to pass through good quality condoms.

• Through blood to blood contact. This mainly happens through the sharing of injecting equipment among injecting drug users. In the past, before screening was introduced, this also occurred through blood transfusions or from infected blood products such as the Factor VIII used to treat haemophilia. Very rarely, it can happen through occupational accidents amongst healthcare workers, such as needle-stick injuries. Fortunately, follow-up studies have shown that less than 1% of individuals who receive injuries with HIV-contaminated needles become infected.

• Vertically, from an HIV-positive woman to her baby during the course of pregnancy, birth or breastfeeding. The average risk of transmission during pregnancy is in the region of 10 to 15%, although it may be greater if the mother has a high viral load or has developed AIDS. Breastfeeding does carry a risk of transmission, and should be avoided in countries where good alternatives to the mother's milk are available.