Avian influenza, or bird flu, has public health officials on alert after an unprecedented spread in dairy cows in the United States this year. Four dairy workers have also tested positive in the country.
A particularly severe variant of the H5N1 strain has been spreading around the world in animals since 2020, causing lethal outbreaks in commercial poultry and sporadic infections in other species from alpacas to house cats. Until this year, it had never infected cows.
Different bird flu strains have been found in Australia and Mexico in humans, while different H5 subtypes are also present around the world in both animals and humans, in countries including China and Cambodia.
Most of the human cases reported exposure to poultry, live poultry markets, or dairy cattle prior to infection, but scientists are worried the virus could mutate in ways that make it more easily spread from person-to-person, which could spark a pandemic. The World Health Organization says the risk to people is low at this point.