Bird Flu Picks a Genetic Lock

The most frightening aspect of avian flu has always been its astonishing virulence, but the human death rate in hard-hit northern Vietnam has fallen to 34% this year, down from almost 80% for the entire country in 2004. Good news? Not if you're an epidemiologist. Investigators for the World Health Organization (WHO) have raised concerns that even though the H5N1 bird-flu virus appears to be weakening, it may be adapting better to human beingspotentially opening the door to a flu pandemic.

Researchers have found that as the fatality rate dropped in northern Vietnam, there has been an increase in...