A 10th of children have a "monkey-like" immune system that stops them developing Aids, a study suggests.
An untreated HIV infection will kill 60 per cent of children within two and a half years, but the equivalent infection in monkeys is not fatal.
The findings could lead to new immune-based therapies for HIV infection.
The virus eventually wipes out the immune system, leaving the body vulnerable to other infections, what is known as Acquired Human Immunodeficiency Syndrome (Aids).
The researchers analysed the blood of 170 children from South Africa who had HIV, had never had antiretroviral therapy and yet had not developed Aids.
Tests showed they had tens of thousands of human immunodeficiency viruses in every millilitre of their blood.
This would normally send their immune system into overdrive, trying to fight the infection or simply make them seriously ill, but neither had happened.
Prof. Philip Goulder, one of the researchers from the University of Oxford, told the BBC: "Essentially, their immune system is ignoring the virus as far as possible.
"Waging war against the virus is in most cases the wrong thing to do."
Counter-intuitively, not attacking the virus seems to save the immune system.
HIV kills white blood cells - the warriors of the immune system.
And when the body's defences go into overdrive, even more of them can be killed by chronic levels of inflammation.