Influenza is a viral disease that strikes many different types of animals, including birds, pigs, horses, and humans. Canine influenza is considered an emerging disease, and resulted from a mutation in a previously existing equine influenza virus strain known as H3N8. Prior to this development, influenza A virus strains were unable to spread easily within canine populations. After the development of this new canine strain, outbreaks of canine influenza were reported for the first time in greyhound kennels and at greyhound racetracks in 2004. Serological studies indicate that the virus may have been present in greyhounds as early as 1999. Although canine influenza first appeared in greyhounds, it now causes respiratory illness in a variety of breeds, and all dogs are considered to be at risk of infection.