Human immunodeficiency virus (HIV) is the virus that causes acquired immunodeficiency syndrome (AIDS), a condition in which the immune system begins to fail, leading to life-threatening opportunistic infections.  Infection with HIV occurs via the transfer of certain body fluids. The four major routes of transmission are unsafe sex, contaminated needles, breast milk, and transmission from an infected mother to her baby at birth.

From its discovery in 1981 to 2006, AIDS killed more than 25 million people. HIV infects about 0.6% of the world's population. In 2005 alone, AIDS claimed an estimated 2.4–3.3 million lives, of which more than 570,000 were children. A third of these deaths are occurring in Sub-Saharan Africa, retarding economic growth and increasing poverty. According to current estimates, HIV is set to infect 90 million people in Africa, resulting in a minimum estimate of 18 million orphans.

In the UK, Around 83,000 people were living with HIV in the UK at the end of 2008, of whom an estimated 27% were unaware of their infection. In 2009, there were 6,630 new diagnoses of HIV, contributing to a cumulative total of 111,922 cases reported by the end of June 2010.

As of June 2010, there have been 26,262 diagnoses of AIDS in the UK, and 19,457 people diagnosed with HIV have died.