Swine influenza is an acute, highly contagious, respiratory disease that results from infection with type A influenza virus. Pigs are the principal hosts of classic swine influenza virus. Influenza is transmitted primarily pig-to-pig by the nasopharyngeal route. Nasal secretions are laden with virus during the acute febrile stage. The virus is easily carried and spread by avian species, particularly waterfowl, and humans. Care should be taken to prevent spread from and between birds and humans to swine.