Beginning in March, 2013, reports started to come out of Eastern China that a new “bird flu” virus was loose and causing infections in humans. The new virus is called H7N9. Should we in the U.S. be worried? Neither I nor anyone I know can give a confident answer to that question. So far the news is reassuring. A report from the U.S. Centers for Disease Control and Prevention (CDC), and another from a team from China in this week’s New England Journal of Medicine, indicate that so far the new H7N9 virus has not clearly spread from one person to another, and has not spread outside of Eastern China. Even so, every public health agency around the world is keeping a close eye on China. That’s because a virus that cannot spread easily from one person to another can change or swap genes—and suddenly be capable of spreading easily.