Only a few evolved into strains that could easily infect people.

Researchers admitted that they could not easily infect comparable white cells in mice for such tests, but they said the technique appeared to work in another kind of white cell tested in monkeys.

Bell was angry at this demonstration of a pessimistic attitude since it could easily infect the others.

Public health authorities are worried that the H5N1 avian flu decimating birds in Asia and now spreading to birds in Europe could mutate into a form that would easily infect people.

It would create a viable offspring between Ontongard and the native life, a child that could breed with the native stock and produce offspring that the Ontongard could easily infect.

In countries with no public sewerage and running water, the cholera bacteria easily infect local drinking water and streams, contaminating fish and vegetables, and perpetuating their existence.

But scientists are concerned that the virus might adapt to more easily infect people.

"You can easily infect yourself after touching a contaminated surface and then touching your nose or eyes."

In these conditions a virus is more likely to mutate into a form that more easily infects humans.