There are four types of influenza viruses: A, B, C, and D. Wild birds—especially wild ducks, geese, swans, gulls, shorebirds, and swallows—are the natural hosts of most influenza A viruses.

Influenza A viruses, also known as avian influenza, have infected many different species of animals, including ducks, chickens, pigs, whales, horses, and seals. However, some influenza viruses are specific to certain species, with the exception of birds, which are hosts to all known influenza A viruses. So can avian influenza infect humans?