Why doesn't bird flu spread easily between people? Scientists think they've found a reason. The virus prefers to infect cells in the lung instead of areas like the nose and windpipe, so it's not easily coughed or sneezed out into the air, new research says. But that behavior could change if the virus mutates. Experts say the new research doesn't indicate how likely the virus is to change genetically and unleash a worldwide outbreak of lethal flu. However, the work suggests one of the signs to watch for in new virus samples to help gauge the danger to humans. The work, reported in Thursday's issue of the journal Nature, comes from University of Wisconsin-Madison virologist Yoshihiro Kawaoka with colleagues in Japan. Similar results, from the Erasmus Medical Center in Rotterdam, the Netherlands, will be published online Thursday by the journal Science.