Even as the United States grapples with an outbreak of H5N1 flu in dairy cattle, the World Health Organization has announced the first known human infection with a different strain, H5N2, in a person in Mexico who died of complications of their infection.

Both of these strains of the influenza A virus have primarily affected farmed and wild birds, infecting flocks around the world. About 900 human cases of H5N1 have been reported since 1996, and about half of people known to be infected have died. On Wednesday, WHO said it had confirmed the first case of human infection of H5N2 in a 59-year-old resident of Mexico.

The source of the person’s infection is unknown, although WHO noted that H5N2 viruses had been reported in poultry in Mexico.

Because any new influenza A infection in people has the potential for high public health impact, it must be reported to WHO. The organization said that based on current knowledge, the risk to the general public from H5N2 is low, since other H5 subtypes are known to infect humans but none has been shown to cause sustained human-to-human spread.

The person, who had multiple underlying medical conditions, had been bedridden for three weeks for other reasons before they developed new symptoms. In mid-April, they developed fever, shortness of breath, diarrhea, nausea and weakness. On April 24, they sought medical attention and were hospitalized at the National Institute of Respiratory Diseases in Mexico City. The person died of complications that day.

On May 8, a sample sent for specialized testing indicated that it was positive for H5N2, and the result was subsequently confirmed by a different lab.

Of 17 people at the hospital who’d been in contact with the patient, no further cases were identified. Twelve additional contacts were identified near the person’s residence, and seven of those had symptoms, but testing of samples from these cases didn’t identify Covid-19 or any type of flu. Scientists are now testing blood samples to see whether they can find antibodies that would point to past infections.

In March, an H5N2 outbreak was reported in a backyard poultry farm in a neighboring state to the one where the person lived. According to WHO, it hasn’t been possible to establish whether this case is related to recent poultry outbreaks.

Flu viruses normally circulate in birds but occasionally can jump to other species, including humans. People catch the viruses from direct contact with infected animals or contaminated environments.

Depending on the original host, type A flu strains can be classified as bird flu, swine flu or other types of animal influenza viruses.

Bird flu infections in humans can cause mild to severe upper respiratory tract infections and can be fatal.  Eye infections like conjunctivitis, gut symptoms and brain swelling are also possible.