The worldwide AIDS pandemic is largely caused by one HIV strain called HIV-1, which after a variable length of time leads to AIDS and death in most infected people. There is a second HIV strain called HIV-2, which is mostly found in West Africa. This virus behaves quite differently from HIV-1: some HIV-2-infeceted people develop AIDS just as if they had HIV-1 but others, as many as 80%, have a normal lifespan and die of old age instead. We want to see if the immune response to the virus is different in these two groups of HIV-2-infected people, which may give useful clues about how to develop an HIV vaccine. We will be looking predominantly at cells in the blood called cytotoxic (or killer) lymphocytes because these cells have the ability to recognise and destroy HIV-infected cells.