People, pigs, and birds aren’t alone in their influenza woes. Horses, whales, and seals catch the virus, but scientists think all flu is actually avian flu, explains Brian Palmer in Slate. Birds carry each of the 144 types of influenza, which is able to jump species by mutating to fit cell receptors in other animals; pig cells in particular are easy targets.
Cats, dogs, and ferrets can also catch influenza, but the new strains can’t spread quickly enough to survive multiple generations and be classified a new distinct disease. Why are birds, pigs, and people hot carriers? In the case of the latter two, experts suspect close living quarters. And since most strains don’t make birds sick, the virus has time to develop and be passed on, often by migratory fowl.