Swine influenza or swine flu (commonly misspelled swine flue) refers to a range of influenza cases that are caused by viruses endemic to pigs. Swine flu infects some people every year, typically those in close contact with pigs. The influenza virus can be carried by migratory birds, who are not usually affected. The bird droppings can infect pigs, where the virus may mutate. New strains may be able to infect humans, which is what led to the 2009 H1N1 outbreak.