Tracking the bird flu, experts see a familiar threat — and a virus whose course is hard to predict

(STAT) Veteran influenza epidemiologist Keiji Fukuda remembers vividly when he first became fearful that a virulent bird flu virus, H5N1, might be on the verge of triggering a devastating pandemic. The virus, seemingly out of nowhere, did something bird flu viruses were thought not to be able to do. It infected 18 people, killing six of them. That happened in 1997, in Hong Kong. A quarter century later, H5N1 has returned to the headlines, with an outbreak at a Spanish mink farm — reported in mid-January — triggering the latest round of fears that the virus might be inching closer to acquiring the ability to easily transmit among humans.