HIV stands for human immunodeficiency virus. HIV was originally isolated in Paris in May 1983 by Luc Montagnier and belongs to a group of viruses called retroviruses.

Viruses copy their genetic material into the genetic material of human cells. This means that infected cells stay infected for the rest of their lives.

Through mechanisms that are still not fully understood, HIV prevents the immune system from working properly. Normally, the body's immune system would fight off infection. However, HIV is able to infect CD4 T-cells, key cells that coordinate the immune system's fight against infection. Many CD4 T-cells are actually destroyed by being infected; others, including CD4 T-cells which are not themselves infected, no longer work properly.