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To understand how the virus eventually causes AIDS, see the section “Course of the disease”.

For detailed discussion of evidence that HIV causes AIDS, go to http://www.niaid.nih.gov/factsheets/evidhiv.htm

Viruses that are very closely related to HIV are found in other primates (apes and monkeys). These viruses are called Simian Immunodeficiency Viruses (SIV). HIV-2 is genetically almost indistinguishable from the SIV found in sooty mangabeys. A very close genetic relative of HIV-1 has been found in chimpanzees. Therefore most scientists accept that the human immunodeficiency viruses are recently derived from these primate viruses. The earliest blood sample found to contain HIV dates from 1959; this sample was collected in central Africa.

Based on molecular technology and the use of large computer programmes, scientists have been able to trace back the genetic origins of HIV-1 and HIV-2 and roughly pinpoint the time when these viruses first appeared in humans. The current theory is that sometime between 1930-1940 there was a “species-jump” of certain SIV’s into human populations, probably through the practise of slaughtering, preparing and consuming of “bush meat” from monkeys in parts of Central and West Africa.

HIV is not as contagious as is often believed. The virus does not survive long outside the body and can only be transmitted through the direct exchange of certain body fluids such as blood, semen and vaginal fluid. The virus can gain access to the body at its moist surfaces ("mucous membranes") during sex, or through direct injection into the blood stream. Sex is the major mode of transmission of HIV worldwide.

HIV can be transferred from one person to another (transmitted) through:

• Unprotected vaginal or anal intercourse with an infected person
• A mother’s infection passing to her child during pregnancy, birth or breastfeeding (called vertical transmission) – the risk of HIV passing from mother to child is approximately 30%
• Injection with contaminated needles, which may occur when intravenous drug users share needles, or when health care workers are involved in needleprick accidents
• Use of contaminated surgical instruments, for example during traditional circumcision
• Blood transfusion with infected blood
• Contact of a mucous-membrane surface with infected blood or body fluid, for example with a splash in the eye (Note that the virus cannot penetrate undamaged skin.)

If a person is exposed to HIV in one of the above ways, infection is not inevitable. The likelihood of transmission of HIV is determined by factors such as the concentration of HIV present in the body fluids. For example, although HIV has been detected in saliva, the concentration is thought to be too low for HIV to be transmitted through deep/wet kissing since it would require the exchange of almost one litre of saliva between individuals before there would be sufficient virus available for possible transmission. Additionally, a digestive protein in human saliva tends to inactivate the virus.

The risk of HIV transmission also depends on the stage of infection the HIV-positive sexual partner is in. Virus concentrations in blood and body fluids are highest when a person has very recently been infected with HIV, or otherwise very late in the disease, when AIDS has developed. Very early after infection the virus can multiply rapidly as the immune system has not had time to respond and fight back, and late in the disease the virus can multiply rapidly because it has destroyed the immune system altogether. However, it is important to note that once a person is infected with HIV, their blood, semen or vaginal fluids are always infectious, for the rest of their lives.

Vulnerability to HIV infection through sexual contact is increased if a person has sores on the genitals, mouth or around the anus/rectum. These sores can be caused by rough intercourse, other sexually transmitted diseases (STDs), gum disease or overuse of spermicides.

In heterosexual sex, women are more vulnerable to HIV infection because of the large mucous-membrane surface area of the vagina compared to that of the urethra (penile opening). Therefore, in regions where heterosexual sex is the main way HIV is transmitted (as in South Africa), approximately four women are infected for every three men that are infected.

Men who are circumcised have a significantly lower risk of being infected with HIV.

Fortunately, people can take action to reduce their risk of infection. For example, a person who uses a condom every time he or she has sex is at far lower risk of infection than someone who has unprotected sex.

The following outlines common sexual behaviours according to relative risk:

Very low risk

• Masturbation (including mutual masturbation)
• Oral sex on a man with a condom
• Oral sex on a woman with a barrier method (such as plastic wrap, dental dam or a condom cut open)

Low risk

• Wet/deep kissing (when sores or gum disease, and therefore blood, are present)
• Oral sex
• Vaginal sex with a male or female condom
• Anal sex with a male or female condom

High risk

• Anal intercourse without a condom
• Vaginal sex without a condom

The receptive partner (vaginal or anal intercourse) is at much greater risk of being infected from an HIV-positive partner than vice versa.

How HIV is not transmitted

• Mosquito bites
• Urine or sweat
• Public toilets, saunas, showers or swimming pools
• Sharing towels, linen or clothing
• Going to school with, socialising or working with HIV-positive people
• Sharing cutlery or crockery
• Sneezes or coughs
• Touching or hugging
• Kissing a person with HIV (when no blood is exchanged through cuts or sores
• (Sexual) contact with animals, since HIV is strictly a human virus and is not carried by animals

In South Africa, blood donated for transfusions or blood products is screened for antibodies to HIV and for the presence of one of the viral proteins. In 2005, blood transfusion services in South Africa introduced a test for HIV RNA (the genetic material of the virus) further improving the safety of donated blood. Any contaminated blood is discarded. The probability of HIV infection via blood transfusion in this country is therefore extremely low, but can theoretically still occur because the tests used do not detect very early HIV infection (the first few days) in a donor.

Reviewed by Dr Craig Corcoran, Clinical Virologist, National Health Laboratory Service and University of Cape Town, October 2006.

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