Scientists are on alert for signs the virus is adapting to spread more easily among humans, after the WHO announced the death from April

The World Health Organization (WHO) has said 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 WHO said Wednesday it wasn’t clear how the person became infected. “Although the source of exposure to the virus in this case is currently unknown, A(H5N2) viruses have been reported in poultry in Mexico,” WHO said in a statement.

Scientists are on alert for changes in the virus that could signal that bird flu is adapting to spread more easily among humans.