A man’s death in Mexico was caused by a strain of bird flu called H5N2 that has never before been found in a human, the World Health Organization said Wednesday.

The WHO said it wasn’t clear how the man became infected, although H5N2 has been reported in poultry in Mexico.

There are numerous types of bird flu. H5N2 is not the same strain that has infected multiple dairy cow herds in the US That strain is called H5N1 and three farmworkers have gotten mild infections.

Other bird flu varieties have killed people across the world in previous years, including 18 people in China during an outbreak of H5N6 in 2021, according to a timeline of bird flu outbreaks from the US Centers for Disease Control and Prevention.

Mexican health officials alerted the WHO that a 59-year-old man who died in a Mexico City hospital had the virus despite no known exposure to poultry or other animals.

According to family members, the WHO release said, the patient had been bedridden for unrelated reasons before developing a fever, shortness of breath and diarrhea on April 17. Mexico’s public health department said in a statement that he had underlying ailments, including chronic kidney failure, diabetes and high blood pressure.

Hospital care was sought on April 24 and the man died the same day.

Initial tests showed an unidentified type of flu that subsequent weeks of lab testing confirmed was H5N2.

The WHO said the risk to people in Mexico is low, and that no further human cases have been discovered so far despite testing people who came in contact with the deceased at home and in the hospital.

There had been three poultry outbreaks of H5N2 in nearby parts of Mexico in March but authorities haven’t been able to find a connection. Mexican officials also are monitoring birds near a shallow lake on the outskirts of Mexico City.

Whenever bird flu circulates in poultry, there is a risk that people in close contact with flocks can become infected. Health authorities are closely watching for any signs that the viruses are evolving to spread easily from person to person, and experts are concerned as more mammal species contract bird flu viruses.