On Jan. 27, 2025, researchers reported that the H5N1 virus is adapting to new mammalian hosts, raising the possibility of the virus spreading between humans. Ten months on from the shocking discovery that a virus usually carried by wild birds can readily infect cows, at least 68 people in North America have become ill from the pathogen and one person has died.

Although many of the infections have been mild, emerging data indicate that variants of the avian influenza virus H5N1 that are spreading in North America can cause severe disease and death, especially when passed directly to humans from birds. The virus is also adapting to new hosts — cows and other mammals — raising the risk that it could spark a human pandemic.

There are two main variants of H5N1 that researchers are monitoring: one, called B3.13, is spreading mainly in cows; the other, called D1.1, is found mostly in wild and domesticated birds, including chickens raised for poultry. B3.13 has spread rapidly in cattle across the United States, infecting more than 900 herds across 16 states, and has also infected other animals, such as cats, skunks and poultry.

At least 24 people have become ill after exposure to sick birds, and 2 of these infections, caused by D1.1, were severe — one person was in hospital for months and the other died. These numbers are too small to enable researchers to determine whether one variant of the virus is more dangerous than the other, says Seema Lakdawala, an influenza virologist at Emory University School of Medicine in Atlanta, Georgia.

Dairy workers are vulnerable to infection because, during the milking process, they can inhale airborne milk particles and milk droplets can splash into their eyes. Some data suggest that if the virus enters the lungs directly, it could cause a severe infection. The study was published in Nature.