HIV (human immunodeficiency virus) is a virus that attacks and breaks down the body's immune system --the "internal defense force" that fights off infections and disease. When the immune system becomes weak, we lose our protection against illness and can develop many serious, often deadly, infections and cancers.

AIDS (acquired immune deficiency syndrome) is the name for the condition people develop after getting one or more of the serious infections connected with HIV, or when blood tests show that their immune system has been very badly damaged by the virus.

It usually takes many years for HIV to break down a person's immune system and cause AIDS. Most people have few, if any, symptoms for several years after they get infected. But once HIV gets into the body, it can "hide" for months or years, and during this time it may be doing serious damage to the immune system. People who appear perfectly healthy may have the virus, without knowing it, and pass it on to others. Scientists still don't know exactly how long it takes for a person to become sick and develop AIDS after getting infected with HIV.