Avian influenza or bird flu is a disease that naturally spreads among wild aquatic birds worldwide, as well as domestic poultry and other bird and animal species. Health officials on Thursday said they don’t know how a Missouri person caught bird flu but believe it may be a rare instance of a “one-off" standalone illness.

Bird flu has been detected in over 200 dairy herds in 14 states, but not in Missouri, according to the CDC. Bird flu has also been found in commercial and backyard flocks and in wild birds. This was the first bird flu case detected through routine influenza surveillance, officials noted.

Missouri hasn’t reported any infections in cows. However, farmers in many places have refused to test their cows. So there could be a lot more of the virus in the country than is currently known. The virus causing the outbreak in cows is highly pathogenic avian influenza — sometimes shortened to HPAI — of the H5N1 subtype.