HIV stands for Human Immunodeficiency Virus. Someone who is infected with HIV is said to be HIV positive.

The virus was originally isolated in Paris in May 1983 by Luc Montagnier. It belongs to a group of viruses called retroviruses.

The virus copies its genetic material into the genetic material of human cells. This means that infected cells remain infected for the rest of their lives.

Through mechanisms that are still not fully understood, HIV damages the immune system by infecting CD4 cells, which normally coordinate immune responses. Many are destroyed by being infected, while others no longer work properly.