Influenza: the once and future pandemic.

Influenza A viruses infect large numbers of warm-blooded animals, including wild birds, domestic birds, pigs, horses, and humans. Influenza viruses can switch hosts to form new lineages in novel hosts. The most significant of these events is the emergence of antigenically novel influenza A viruses in humans, leading to pandemics. Influenza pandemics have been reported for at least 500 years, with inter-pandemic intervals averaging approximately 40 years.