How does HIV disable the very cells that run your defences?

HIV targets and kills CD4+ T helper cells, the coordinator cells that normally tell killer T cells, B cells and phagocytes what to attack. As HIV destroys growing numbers of these cells, the body gradually loses the ability to mount a coordinated defense. Once CD4+ T cell counts fall below a critical level, cell-mediated immunity is lost and the body becomes vulnerable to infections it would normally handle easily, a stage known as AIDS.

HIV enters a CD4+ T cell by using a surface protein to latch onto the CD4 receptor itself, the very molecule that gives the cell its name, then hijacks the cell to copy itself; the infected cell usually dies. Without treatment, the infection can progress over many years, eventually weakening the immune system enough for opportunistic infections and tumors rare in healthy people to appear. Average life expectancy without treatment is estimated at around eleven years.

With antiretroviral treatment, the virus can be suppressed to an undetectable level in the blood. Someone who starts treatment early and stays on it can expect a near-normal lifespan, and an undetectable viral load carries effectively no risk of sexual transmission, summarized by public health campaigns as Undetectable equals Untransmittable.