Few people had heard of HIV or AIDS before the 1980s. That all changed around 1982.
People learned that an infection called HIV (human immunodeficiency virus) can lead to AIDS (acquired immune deficiency syndrome), a life-threatening illness that makes people vulnerable to infections and cancers.
Genetic researchers believe that HIV originated in west equatorial Africa and was passed from chimpanzees to humans during the late 19th or early 20th century.
There are many theories as to why and how HIV/AIDS adapted from an animal disease into one affecting humans, but no one has the full answer yet.
At first, many people died because there were no drugs available to combat HIV or AIDS. Today, there are effective drugs that reduce the impact of the disease, although the drugs are expensive and not affordable in poorer countries.
To date, more than 30 million people around the world have died of AIDS-related illnesses. In 2009, nine countries in Africa had more than one-tenth of their populations aged 15 to 49 infected with HIV. HIV/AIDS remains a major world health problem.