The latest version is called H6N1, and represents the first time that this strain of bird flu has jumped from birds to people.
Flu researchers are especially wary of birds, from wild avian species like migrating geese to run-of-the-mill chickens at local poultry markets. They harbor a series of influenza strains that generally don’t make the birds sick, but could cause serious disease in people if if they jumped to human hosts.
In recent years, more bird flu viruses that had never infected people before have been finding new human hosts. Last spring, for example, scientists in China reported the first human cases of H7N9 infections. These viruses previously circulated among birds, but mutations helped them to survive and sicken people as well.