Influenza can be a weapon of mass destruction. The latest candidate worth worrying about is H7N9. It jumped from birds to humans earlier this year. Of the 137 people known to have been infected, 45 died. There's no evidence that H7N9 spreads among humans. On the other hand, it wouldn't surprise any of the experts if H7N9 learns how to jump from person to person and mutates into a fast-moving mass murderer.