Another case of a mysterious avian flu was confirmed in Hong Kong Tuesday and at least three more are suspected, increasing the likelihood of human-to-human transmission that could signal a worldwide flu epidemic.

The virus is thought to have passed from chickens to people, but these new cases increase concern that the virus might have adapted to a form that passes from person to person. If that happens, scientists say, it could spread rapidly.

Officials at the U.S. Centers for Disease Control and Prevention say there are now seven confirmed cases of the new flu, and two deaths. So far, no cases have been detected outside Hong Kong.