HIV is the human immunodeficiency virus that causes acquired immunodeficiency syndrome, or AIDS. Its transmission is by exchange of bodily fluids. HIV can only enter immune cells with the surface protein gp120. The virus can hide in these cells for many years before it is activated, although it can be transmitted throughout this period. Once activated, the virus begins to replicate, ultimately causing the immune system of the infected person to collapse making them vulnerable to opportunistic infections. ‘HIV’ describes how evolutionary biology has been used to clarify the origins of the epidemic. The rapid mutation rates and recombination that make HIV very hard to treat are also explained. Despite these challenges, a regimen of highly active anti-retroviral therapies (HAART), developed in the mid 1990s, is extraordinarily effective against HIV.