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Undetectable viral load and infectiousness
An undetectable HIV viral load is the goal of anti-HIV treatment. This does not mean that you have been been cured of HIV, but that the combination of drugs you are taking has reduced HIV's ability to reproduce so it can no longer be detected in the blood.
An undetectable viral load in blood does not necessarily mean that you are not infectious.
Although many people with undetectable viral loads in their blood also have an undetectable viral load in their sexual fluid and seem less likely to transmit HIV, this is not always the case. Some people with undetectable viral load in their blood have sufficient viral load in their sexual fluids to infect somebody else.
Studies have mainly been conducted in men, and these have found that having an untreated STI, particularly gonorrhoea, increases the chances that HIV viral load will be detectable in semen.
HIV can also be present in cells and it is possible that these could transmit HIV infection even when a person has an undetectable viral load.
In addition, studies have also found that men with high blood viral loads have very high viral loads in their semen and are very infectious.
If a person is resistant to anti-HIV drugs, it is thought they can infect other people with drug resistant HIV, and about 10% of new HIV-infections in the UK are with drug resistant virus. This means that the person newly infected with HIV has limited treatment options before they have taken a single anti-HIV drug.