First the good news: bird flu is becoming less deadly. Now the bad: scientists fear that this is the very thing that could make the virus more able to cause a pandemic that would kill hundreds of millions of people.

This paradox – emerging from Egypt, the most recent epicentre of the disease – threatens to increase the disease's ability to spread from person to person by helping it achieve the crucial mutation in the virus which could turn it into the greatest plague to hit Britain since the Black Death. Last year the Government identified the bird-flu virus, codenamed H5N1, as the biggest threat facing the country – with the potential to kill up to 750,000 Britons.

The World Health Organisation is to back an investigation into a change in the pattern of the disease in Egypt, the most seriously affected country outside Asia. Although infections have been on the rise this year, with three more reported last week, they have almost all been in children under the age of three, while 12 months ago it was mainly adults and older children who were affected. And the infections have been much milder than usual; the disease normally kills more than half of those affected; all of the 11 Egyptians so far infected this year are still alive.

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Experts say that these developments make it more likely that the virus will spread. Ironically, its very virulence has provided an important safeguard. It did not get much chance to infect other people when it killed its victims swiftly, but now it has much more of a chance to mutate and be passed on.

The WHO fears that this year's rise in infections among small children, without similar cases being seen in older people, raises questions about whether adults are being infected but not falling ill, so acting as symptomless carriers of the disease. Its investigation, due to start this summer, will see if this is happening by testing the blood of people who may have been in contact with infected birds, but who have not themselves become sick.

John Jabbour, who works with WHO in Cairo, told Reuters last week: "There is something strange happening in Egypt. Why in children now and not in adults? We need to see if there are sub-clinical cases in the community." He added that if the research did find such cases, they would be the first to be discovered anywhere in the world.

Though he stressed that there was still no evidence of the disease passing from person to person, other experts are also becoming alarmed. Professor Robert Webster, of St Jude Children's Research Hospital in Memphis, Tennessee – who is the world's leading authority on the disease – told The Independent on Sunday that, while he himself had not seen firm data, the WHO in Egypt was raising "a very, very important issue" which should receive "maximum attention". He added: "I hope to hell they are wrong. If this damn thing becomes less pathogenic, it will become more transmissible."

And Professor John Oxford, of Queen Mary, University of London, said that any evidence that H5N1 was becoming less deadly would be serious, as the greatest cause for concern was the disease's ability to spread.

Even a much less virulent strain of the virus could result in a devastating pandemic. Studies show that an outbreak that killed as few as 5 per cent of those it infected could still cause hundreds of millions of deaths around the world.