Mellowing in old age? At the centre of this image are three cross-sectioned viruses, their spiked protein coats coloured yellow and green, and their triangular core of RNA genetic material coloured red (Image: Eye of Science/Science Photo Library)

HIV seems to be slowly becoming less aggressive in parts of Africa. The change so far has been small, but if it continues the disease could become substantially less dangerous.

It was once thought that pathogens always evolved to become less deadly, so that their hosts have more chance of surviving and spreading the disease. Now we know there can also be evolutionary pressures in the opposite direction, with the net result probably depending on many factors that are different for each disease.

The changes in HIV so far have been relatively subtle – equivalent to people developing AIDS about 2.5 years later than they did at the start of the epidemic.


HIV seems to have first crossed over from chimpanzees to humans in West Africa about 100 years ago, before growing in the last 30 years into a global pandemic that has now killed as many as 40 million people.

To track how HIV has been evolving, Philip Goulder of the University of Oxford and his colleagues compared HIV samples taken from 842 pregnant women in Botswana and South Africa. In Botswana, the epidemic took off in the mid-1980s, compared with the mid-90s in South Africa – so HIV in Botswana has had about a decade longer to evolve.

When tested on cells grown in a lab, the HIV from Botswana reproduced more slowly than that from South Africa, which should mean it takes longer to destroy people’s immune systems and result in AIDS.

“To show it’s adapting so rapidly is very significant,” says José Borghans of the University Medical Center Utrecht in the Netherlands.

Surprising side effect

One reason for the change could be the growing use of HIV drugs, says Goulder. People with the most virulent form of the virus get sick sooner and start drug treatment. This reduces the level of the virus in their blood and sexual fluids almost to zero, so they are unlikely to pass it on. This means that a more aggressive virus is less likely to be transmitted.

“It’s a benefit of therapy that nobody thought of,” says Goulder. “That’s another reason to provide it.”

Those accessing HIV treatment have increased in recent years, rising from just 5 million in 2010 to 13.6 million people this June.

But there seems to be another factor at play too – the virus is responding to people whose immune systems are naturally better at keeping the infection under control for longer.

About 15 per cent of people in southern Africa have genes that mean their immune cells are good at recognising and targeting crucial proteins belonging to the virus. In such people, HIV can only survive by mutating those proteins to evade detection, which makes it slower at reproducing. When those people pass on the virus, it retains that weakness. “The viruses that are left are the ones that are least able to cause disease,” says Goulder.

His team found that in Botswana about half the viruses sampled had these mutations, while in South Africa about 40 per cent did.

Conflicting evidence

While these findings are encouraging, this doesn’t mean we can assume that the same is happening in Europe and the US, says Borghans. A variety of studies in the West have found conflicting results, suggesting variously that HIV is either becoming more virulent, stable or slowing down.

Goulder says most previous studies didn’t measure the virus’s replication directly, but used proxies such as the amount of the virus in people’s blood, making these studies potentially less accurate and difficult to compare.

He adds that HIV would be expected to evolve faster in sub-Saharan Africa than in the West because infection rates are much higher there. “You would expect to see the major differences to viral replicative capacity where the epidemic is concentrated,” he says. “This is good news.”

Journal reference: PNAS, DOI: 10.1073/pnas.1413339111