By Roland Pease
BBC science correspondent
Bird flu could readily mutate into a pandemic form if it infects people with Aids, a flu expert has warned.
It is feared bird flu will jump from human to human
Dr Robert Webster said it was possible people with Aids, who have depressed immune systems, could harbour the deadly H5N1 strain of bird flu.
This would potentially give it the opportunity to become better adapted - and more dangerous - to humans.
Dr Webster was speaking at a conference organised by the Council on Foreign Relations in New York.
At present, H5N1 cannot pass easily from human to human. It has so far infected around 125 people in South East Asia, but most of these have had close contact with infected birds.
Experts fear that the widespread infection of birds in this region, coupled with the close mixing of birds and people, could lead to the virus evolving to pose a more deadly threat.
But Dr Webster, of St Jude Children's Research Hospital im Memphis, said the key could be when H5N1 reaches East Africa, where HIV/Aids is rife.
Cannot clear virus
He said experience with immune-compromised cancer patients at his hospital had showed they are unable to clear normal flu virus from their systems, and can shed copies of the virus for weeks.
The same could be expected of AIDS patients coming down with H5N1, he said.
"We're all very worried by the prospect," he told the BBC.
Reproducing over a long period inside a human would be the ideal conditions for more infectious forms of the virus to develop.
H5N1 has not reached East Africa yet, but it is the final destination for many birds currently migrating from infected areas.
Officials at the UN Food and Agriculture Organisation expect to arrive there soon.
They believe that because the social conditions are close to those in Asia, and farming practices are similar, the virus could take a grip among poultry as it has in Vietnam, Thailand, Indonesia and China.
Health expert Laurie Garrett adds that with malaria, tuberculosis and HIV already widespread in Africa, it will be difficult to single out the symptoms of bird flu in new victims - high fever and nausea.
The situation is compounded by the parlous state of the health systems across the continent.
The direct effect of H5N1 on people with Aids is hard to predict.
The H5N1 virus overstimulates the immune system, and many of its powerful effects are caused by what medical expert call a "cytokine storm", after the immune molecules excited by the disease.
It was the cytokine storm that overwhelmed so many victims of the 1918 flu pandemic. Aids patients may be spared that fate.
But equally possible, with their immune defences down, they could succumb easily to the disease.
"In that situation," said Laurie Garrett, "vast populations of HIV positive people could be obliterated by the pandemic flu."