Laboratory tests showed that the older viruses were significantly "fitter." They multiplied more easily and were better able to resist anti-retroviral drugs.

The findings appear to contradict recent trends which indicate a growth in HIV drug resistance. But they support theories which suggest that viruses sometimes evolve to become less virulent in order to safeguard their survival.

Infection Rates

In this respect, HIV may be following in the footsteps of the virus which causes myxomatosis in rabbits.

When the myxoma virus was deliberately introduced as a control measure in Australia in 1950, rabbit populations were decimated. But weakened strains of the virus quickly emerged, so that many rabbits now develop a chronic form of the disease instead of dying.

Experts warn, however, that HIV infection rates are continuing to rise in the UK and elsewhere, and there should be no scaling down of efforts to curb its spread.

Replicative Fitness

The researchers, led by Kevin Arien from the Institute of Tropical Medicine in Antwerp, looked at HIV-1-infected cells obtained from patients in 1986-89 and 2002-03.

The older viruses out-competed the new ones on 176 occasions out of 238. In nine out of 12 specially controlled and carefully matched tests, the 1980s viruses proved the stronger.

The researchers wrote in the journal AIDS: "These findings suggest that HIV-1 replicative fitness may have decreased in the human population since the start of the pandemic."

Symbiotic Existence

Previous models simulating the spread of infective agents have suggested that many lethal viruses and bacteria may evolve away their virulence, even to the point of "symbiotic existence" where they actually benefit the host.

If a virus is too deadly, it risks working against itself by killing off many of its potential hosts. Becoming less prolific may also help to shield a virus from the host's immune system.