Chinese and U.S. scientists have found that the strain of bird flu that killed 23 people in Asia earlier this year is growing more dangerous over time (Richard Black, BBC Online, June 29).

According to the scientists, who published their research findings in yesterday's issue of Proceedings of The National Academy of Sciences, tests in mice found that samples of the H5N1 virus collected from ducks in China in 1999-2000 were less harmful to mammals than those gathered in 2001-2002.

"Our results demonstrate that while circulating in domestic ducks, H5N1 viruses gradually acquired the characteristics that make them lethal in mice," the scientists reported.

How the virus evolved to become more dangerous to mammals is still not known. One theory is that it happened on farms where pigs and ducks live in close quarters, which allowed the virus to move back and forth between mammals and fowl.

The head of the World Health Organization's global influenza program, Klaus Stohr, said the new study confirms that the bird flu virus is evolving rapidly and says that while there is no need to panic, H5N1 could eventually pose a major threat to humans.

According to scientists, the danger could develop in two ways. The virus could mutate and accumulate enough genetic changes to become able to pass between humans, or it could combine with a highly infectious human flu virus in someone's body, creating a hybrid as deadly as the bird strain and as contagious as a regular human strain (Randolph Schmid, Associated Press/Atlanta Journal-Constitution, June 29).