Health experts have been sounding the alarm about the potential pandemic threat posed by bird flu, which has been showing signs of mutating as it spreads among cows and infects people in the United States.
There is no guarantee that bird flu will ever begin transmitting between humans, and U.S. health authorities have emphasized that the risk to the general public remains low.
The deadly bird flu variant H5N1 first emerged in China in 1996, but over the last four years it has spread more widely than ever before, reaching previously untouched regions such as penguin-haven Antarctica.