HIV is found in the blood and sexual fluids of an infected person, and in the breast milk of an infected woman. There are various ways a person can become infected with HIV.
Injecting drug use
People who inject drugs are vulnerable to infection because in many parts of the world injecting equipment is shared, often because it is illegal to possess them. A tiny amount of blood can transmit HIV, and can be injected directly into the bloodstream with the drugs.
Having unprotected sexual intercourse with an infected person
Sexual intercourse without a condom is risky, because the virus, which is present in the semen and vaginal fluids of people with HIV, can pass directly into the body of their partner. This is more likely where there is broken skin or if someone already has another sexually transmitted infection.
Mother to baby transmission
HIV can be passed from a mother to her baby in the womb, during childbirth and via breast milk. There are drugs and techniques that greatly reduce the chances of this happening, but they are often unavailable in much of the developing world.
Use of infected blood products
In the past many people have been infected with HIV through blood transfusions. In much of the world this is no longer a risk, as blood donations are routinely tested.
Contact with an infected person's blood
If sufficient blood from someone with HIV enters the body of an uninfected person, it can pass the virus on to them.
It is NOT possible to become infected through touching, hugging, sharing crockery and cutlery with, shaking hands with or eating food prepared by someone with HIV.
It is also NOT transmitted by insect or animal bites or from toilet seats.
(China.org.cn November 30, 2004)
