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Infectiousness
If you have HIV, you can potentially pass on the virus to other people. Your blood, semen or vaginal fluid may contain enough HIV to infect another person. There is a risk of passing on HIV only if one of these fluids gets into someone else's body, either directly into their bloodstream or via a mucous membrane in the anus, genitals or less often, the mouth and throat.
HIV transmission only happens in a small minority of the occasions in which an uninfected person is exposed to HIV-infected body fluids. Factors that may influence the risk of HIV transmission during such a risk exposure include:
- the amount of HIV in the infected person's body fluids (i.e. the viral load)
- the type of risk exposure. Sharing drug injecting equipment probably carries the greatest risk. Unprotected anal sex appears to be more risky than unprotected vaginal sex. Sexual activity which damages the body tissues may also increase the risk of HIV transmission if those tissues are exposed to infected fluids
- the presence of other sexually transmitted diseases. These may increase the amount of HIV in the genital fluids of the HIV-infected partner, and/or cause ulcers or inflammation of the mucous membranes that make it easier for HIV to enter the body of the uninfected partner
- inherited genetic factors that may make the uninfected person less susceptible to HIV infection (although no-one appears to be completely immune)
Viral load and infectiousness
People with advanced HIV disease are more infectious than people with early HIV infection. This is thought to be due to the fact that people with more advanced disease usually have higher levels of HIV in their blood and sexual fluids. Likewise, HIV-infected pregnant women with high viral load are more likely to pass on the virus to their infant before or during birth, or by breast-feeding, than mothers with low viral load.
Newly infected people, especially those who are in the `window period' during which they have not yet developed antibodies against HIV, often have very high levels of HIV in their blood. It is possible that they also have high levels of HIV in their sexual fluids. This may mean that risky contact with a newly infected person carries an increased risk of HIV transmission.
The effects of anti-HIV drugs
Effective anti-HIV drugs reduce the amount of HIV in the blood, as reflected by reductions in viral load. Studies suggest that anti-HIV therapy also decreases levels of HIV in other parts of the body, including the sexual fluids. However, there is good evidence that people taking anti-HIV drugs can still pass on HIV to other people – even if their viral load has been reduced to `undetectable'.
There are two major reasons why people with very low or `undetectable' viral load may still be infectious to other people:
- viral load may not be reduced to the same extent, or at the same speed, in different parts of the body. It is possible to have `undetectable' viral load in the blood, but detectable viral load in the sexual fluids
- anti-HIV drugs help to block the production of new HIV particles from HIV-infected cells and the infection of new cells, but they do not kill the HIV-infected cells themselves. HIV can hide within human cells for years despite anti-HIV therapy. If HIV-infected cells in the blood or sexual fluids are passed from one person to another, they may transmit HIV infection
It is also recommended that people with undetectable viral loads use a condom when having sex with other HIV-positive people as there is a growing amount of evidence that a person with HIV can be reinfected, or ‘superinfected’ with another strain of the virus, which could have implications for the success of current or future treatment options.
Pregnant women who take anti-HIV drugs are less likely to pass on HIV to their unborn child than those who do not take anti-HIV drugs. This is probably partly because the drugs reduce the amount of HIV in the mother's body fluids, and partly because the drugs enter the body of the unborn child and make it harder for HIV to get established. Again, anti-HIV drugs do not make an HIV-infected woman entirely non-infectious – there have been several cases in which women with `undetectable' viral load have nevertheless given birth to HIV-infected babies.