H7N9, the influenza strain that has killed nine people in China in recent weeks, may be growing more virulent, said Richard Webby of the World Health Organization. "If this is let spread from where it is now, it will evolve further. That's what viruses do," he said. Although human-to-human transmission hasn't been seen, scientists have uncovered signs of genetic diversity in the strain. Researchers continue to try to isolate the environmental sources of infection, but they need more data on the lives of China's birds in markets, on farms and in the wild, according to Maria Zambon, director of the U.K.'s national influenza center.