AIDS wasn't discovered until the early 1980s

AIDS wasn't discovered until the early 1980s, when doctors in the United States noticed clusters of patients suffering from highly unusual diseases. First seen in gay men in New York and California, these illnesses included Kaposi's sarcoma, a rare skin cancer, and a type of lung infection carried by birds.

Soon cases were also detected in intravenous drug users and recipients of blood transfusions. By 1982 the illness had a name—acquired immune deficiency syndrome. AIDS has since killed around 25 million people worldwide, orphaning 12 million children in Africa alone.

AIDS is triggered by a virus acquired through direct contact with infected body fluids. The virus causes an immune deficiency by attacking a type of white blood cell that helps to fight infections. Because this leads to various diseases, not a single illness, AIDS is referred to as a syndrome. The virus is called HIV (human immunodeficiency virus).

Unprotected sex is HIV's main route into humans, where it targets the white blood cell known as CD4. The virus replicates inside, eventually bursting out and flooding the body in the billions. The immune system then kicks in, and the body and the virus wage all-out war. During the height of battle billions of CD4 cells can be destroyed in a single day. As the cell count drops, the immune system begins to fail and opportunistic infections such as tuberculosis take hold.