Baffled scientists first watched a mysterious virus called H5N1 jump from birds to humans a decade ago in Hong Kong, killing six people and forcing the territory to slaughter its entire poultry population. It quieted for a while, but resurfaced in 2003 with even more questions.

Bird flu has since spread to more than 60 countries in Asia, Africa, Europe and the Middle East, killing at least 211 people along with hundreds of millions of birds.

Pakistan and Myanmar reported their first human infections earlier this month.