Following is a factfile ahead of the 18th International AIDS Conference, running in Vienna from July 18-23:
WHAT IS AIDS? Acquired Immune Deficiency Syndrome, which first came to light in 1981, is the term for a progressive weakening of the immune system. The body is exposed to pneumonia, tuberculosis, fungal disease and other opportunistic infections, as well as specific types of cancer, that ultimately are fatal.
WHAT CAUSES IT? The cause of AIDS is the human immunodeficiency virus (HIV), a retrovirus suspected to have leapt the species barrier from animal primates in central Africa. HIV is found in blood, semen, vaginal secretions and breast milk. It enters the body through cuts or sores or by directly infecting cells in the mucous membranes in the genitals or anus. The main modes of infection are unprotected sexual intercourse, shared use of drug syringes, from a mother to her unborn child and transfusion of contaminated blood.
HOW DOES HIV WORK? HIV destroys key white-blood cells called T-helper lymphocytes. It does this by latching onto a molecule known as CD4 on the cell's surface. The virus then penetrates the cell, hijacking its enzyme-making machinery in order to replicate itself. Baby viruses then burst out to infect other T cells.
HOW LONG DOES IT TAKE? The time from initial infection to full-blown AIDS in untreated patients is about nine years, but the time can vary according to the individual. From AIDS to death generally takes between six months and three years.
WHAT IS THE TOLL? At least 25 million people have been killed by AIDS since the disease came to light in 1981. According to the UN agency UNAIDS, at the end of 2008, around 33.4 million people were living with HIV, 22.4 million of them in sub-Saharan Africa. In 2008, 2.7 million people became infected, a 30-percent fall compared with the peak of HIV's spread 12 years earlier.
IS THERE A CURE? There is currently no cure for HIV, but there is a treatment that can suppress the virus to very low levels. Antiretroviral drugs, a combination of two or three medications that inhibit viral reproduction or stop the virus from fusing with the CD4 T cell, are a lifesaver for millions. But they can also have powerful side effects, including diarrhoea, nausea and cramps, and carry a risk of treatment failure. There are more than 20 antiretroviral drugs today. In poorer countries, 9.5 million people are in immediate need of the therapy, but only four million have access to it.
HOW CAN HIV BE PREVENTED? For sexual intercourse, a latex barrier (a male or female condom) is highly effective. Trials in Africa have shown that male circumcision, which removes mucous cells from the foreskin, can roughly halve the risk of infection for the man, although there is no protection for an uninfected woman. Mother-to-child transmission can be combatted by treating the mother-to-be and the newborn with antiretrovirals. Contamination by blood transfusion is now rare in most countries, thanks to screening of blood donations. Needle exchange programmes, in which a used syringe is swapped for a clean one, can help reduce transmission among drug addicts.
WHAT ABOUT A VACCINE? The search for an HIV vaccine is arguably the toughest challenge of all. Trials of prototype vaccines have shown negligible protection or none at all. However, some encouraging work has been achieved in the lab in identifying antibodies - natural frontline troops in the immune system - that could hopefully be primed to recognise the virus. The effort for an HIV-preventing microbicide gel, which could be used in anal or vaginal intercourse, has a similar report card.
SOURCES: UNAIDS; www.avert.org; US National Institutes of Health (NIH); World Health Organisation (WHO); International AIDS Vaccine Initiative (IAVI).