HIV may be getting weaker, but that does not mean it won't kill you, researchers say.

New evidence from a U.S.-Belgium study shows that the AIDS virus is evolving. What seems to be happening is that HIV makes copies of itself — replicates — less rapidly than it used to.

That makes sense for HIV, which jumped from monkeys to humans only in the last century. Many monkey species get HIV-like viruses that don't kill them. HIV may be evolving in that direction, suggests Guido Vanham, MD, PhD, director of virology at the Institute of Tropical Medicine in Antwerp, Belgium.

"The virus is multiplying less rapidly now than 15 years ago," Vanham tells WebMD. "But there is no clinical evidence of HIV becoming less virulent. Doctors do not have the impression that if people are not treated, they will be free of symptoms — let's be clear on that."

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Vanham and colleagues had in their freezer HIV obtained from 12 people infected with HIV from 1986 to 1989. They then found 12 carefully matched people infected in 2002-2003 with the exact same strain and subtype of the virus. None of these people had ever taken anti-HIV drugs when they donated the study samples.

The researchers then pitted the old HIV against the new HIV to see which one grew better in specialized lab tests. Earlier studies have shown that the better HIV grows under these laboratory conditions, the more deadly it is. Sure enough, the older HIV “out-competed recent viruses,” write the researchers.

That, Vanham says, suggests that HIV and humans may be adapting to one another.

"From the point of view of the virus, the most optimal evolution would be that transmission capacity is preserved or increased, and the capacity to kill the host decreases — because the virus has more chances to survive," he says. "That is what we see in many diseases. There is selection for parasites to be easily transmitted and not kill their hosts."

In this regard, monkeys are way ahead of humans.

"The human race is one of the last remaining primate species that has not yet been infected with a virus like this," Vanham says.

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Adapting to Human Hosts

But lots of other retroviruses already have adapted to humans, says Cleveland Clinic AIDS researcher Miguel E. Quiñones-Mateu. Our genes carry RNA from many different retroviruses.

"We have a lot of retroviruses that have been in our genome for millions of years. And they do no harm," Quiñones-Mateu tells WebMD. "What we will see in the future is more and more people with HIV behaving like long-term non-progressors. And eventually the virus will attenuate enough so nobody dies."

The catch: This isn't going to happen right away.

"We have seen only a tendency, a trend," Quiñones-Mateu says. "It could take 100 years, it could take a million years, but that is what is going to happen."

"The question is whether it is thousands of years or decades," Vanham says. "The suggestion of our study is that we are talking about decades. We are already seeing evolution. But this does not mean the virus is so weak that newly infected people will no longer get disease."

Vanham notes that the study looked at only one strain of HIV, in only one country. He says he'll feel more confident about the findings if studies of other HIV subtypes show the same evolutionary trends and if clinical studies detect a lengthening time from HIV infection to AIDS.

Vanham and colleagues' report — and an editorial comment by Quiñones-Mateu — appear in the Oct. 14 issue of the journal AIDS.

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By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Ariën, K.K. AIDS, Oct. 14, 2005; vol 19: pp 1555-1564. Quiñones-Mateu, M.E. AIDS, Oct. 14, 2005; vol 19: pp 1689. Guido Vanham, MD, PhD, director of virology, Institute of Tropical Medicine, Antwerp, Belgium. Miguel E. Quiñones-Mateu, PhD, assistant professor, The Cleveland Clinic and Center for AIDS Research, Case Western Reserve University, Cleveland.