AIDS is the final, life threatening stage of infection with the virus known as HIV-1.
Infection with HIV-1 severely damages the immune system, the body's defense against disease. HIV-1 infects and gradually destroys T-cells and macrophages, both white blood cells that play key roles in protecting humans against infectious disease caused by viruses, bacteria, fungi and other micro-organisms.
Opportunistic infections by organisms normally posing no problem for control by a healthy immune system, can ravage persons with immune systems damaged by HIV-1 infections. Destruction of the immune system occurs over years; the average onset of clinical disease recognized as AIDS occurs after 3-10 years of HIV-1 infection but can be earlier or later.
AIDS in humans was first identified in the US in 1981, but researchers believe that it was present in Central Africa as early as 1959. AIDS is most often transmitted sexually from one person to another but is also transmitted by blood in shared needles (drug users) and through pregnancy and childbirth. Heterosexual activity is the most frequent route of transmission worldwide. According to UNAIDS, over 40 million people are believed to be HIV-infected globally with infection rates continuing to rise.
Viral load is the best indicator of the speed with which an individual will progress to AIDS, as well as the frequency with which an individual will spread infection. An estimated 1% or fewer of those infected have low enough levels of the virus to preclude progression to disease and to not transmit the infection (they are called: long-term non-progressors).