H3N8 is a subtype of the species Influenza A virus (sometimes called bird flu virus). H3N8 is now endemic in birds, horses and dogs.

H3N8 is suspected of causing a human pandemic in either 1889 or 1900. Sources differ; some say the 1889 pandemic was caused by H2N2. The experts also differ on exactly how sure we can be that either were involved.

It was the subtype that was responsible for over one-forth of the flu infections in wild ducks in a 1997 study.