The current strain is descended from the H5N1 virus, which started killing poultry in China in 1996, and then people too. H5N1 exploded across east Asia in 2004 with the poultry trade, and then spread into Europe and Africa in 2006, thanks to migrating birds. Since then, the virus has lurked mainly in poultry, especially flu-vaccinated chickens in Asia that can carry the virus while being immune to it. So far, 452 people have died after catching it from poultry.

But viruses like H5N1 have also been moving with migrating dabbling ducks like mallards, which are usually immune to it. Birds from all over Eurasia mingle in north-central Asia during the summer, swap viruses, then disperse back to Africa, Asia and Europe for the winter. This has recently allowed H5N1 to hybridise with other kinds of flu. “We do not know what is driving the plethora of H5s,” although changes in climate and migration may be involved, says Julio Pinto at the UN Food and Agriculture Organization in Rome.

Turkey culls

As Europe’s poultry farmers fatten up their geese and turkey for Christmas, one hybrid of H5N1 is now giving them sleepless nights.

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