Of course, when a microbiologist takes E. coli out of its usual habitat and grows it in a laboratory, the strain eventually mutates.

There are plenty of experts who say that the strain of avian flu known as A(H5N1) could mutate and spread efficiently among humans.

In fact, if anything, the strain has mutated into something altogether less containable.

If the strain doesn't mutate its surface antigens and we can grow it readily in tissue culture, we could develop a vaccine quite quickly.

This strain mutated after passing from animals to humans in a way that caused it to spread rapidly and become more virulent.

Scientists have also theorized that some strains of the AIDS virus might mutate within the body to become more virulent during the course of infection.

Turns out that the strain of H5N1 that actually broke out almost all had mutated binding proteins.

That vaccine was never fully developed or used, and the strain has mutated since then.

Also, I believe that the paralysis was not so acute, but the strain has obviously mutated in the centuries and become more virulent.

They base their decision on monitoring and educated guesswork about how the strains will mutate.