1.   But, two days later, mutant viruses that were resistant to the drug had appeared.

2.   Ho suspects that the very length and intensity of the struggle ultimately do in the immune system by encouraging the emergence of mutant viruses that can outstrip its defenses.

3.   Some said AIDS was caused by a mutant virus, a sort of Andromeda strain.

4.   Doctors test mutant virus to hunt cancer cells.

5.   Missing even a few doses lets mutant viruses emerge that are resistant to protease inhibitors.

6.   Some older residents shopping at a market Wednesday seemed unperturbed by yet another spate of bird flu, despite the potential of a dangerous mutant virus.