A man is "severely ill" with a virus never before seen in humans and only detected in animals.

The man was taken to hospital with a high fever, confusion and respiratory distress earlier this month and was later confirmed to have H5N5. It is a subtype of avian influenza, also known as bird flu, that is typically carried by ducks and geese as well as other wild birds.

The man remained in hospital as of last week and an investigation into how the man became ill is ongoing. Officials said no one else has tested positive for the H5N5 strain.

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The Washington State Department of Health described the patient as being "older" and having "underlying health conditions." It also noted the man had a "mixed backyard flock of domestic poultry" at his home in Grays Harbour County, in the southwest region of the US state.

Wild birds could also access the property with agency officials commenting either group of birds is "most likely" the source of the virus exposure. There has been no evidence of person-to-person spread, though experts said the evolution of a virus could be "unpredictable."

H5N1 is a more common strain in the US but is still said to be rare. The difference between the strains is a protein that is on the virus' surface.

H5N1 has infected wild bird and domestic poultry that has persisted since 2022. There have been 71 human cases of H5 bird flue recorded in the US since last year.

Most patients had mild symptoms but a patient from Louisiana died in January. In comparison, there were 81 bird flu cases in the UK that were recorded.

Cases had been confirmed in Cumbria, Lincolnshire, Norfolk, Suffolk as well as Scotland and Wales. Symptoms of bird flue include: high fever, a cough, sore throat and body aches.