Figure 1.

The pig as a “mixing vessel” for influenza A viruses. Wild aquatic birds are the natural reservoirs for influenza A virus subtypes H1-H16. Avian influenza A viruses are frequently transmitted to domestic fowl from the natural wild life reservoirs, and also to pigs from domestic fowl. Human and avian influenza A viruses can infect pigs and reassortment can occur in pigs between avian, swine and human influenza A viruses. Influenza A viruses from pigs can also infect humans. Pigs are postulated as the intermediate host, i.e., the “mixing vessel” for influenza A viruses. Avian influenza viruses from domestic fowl or wild aquatic birds occasionally transmit to humans (e.g., H5N1 virus). Solid lines; frequent and/or confirmed transmission events: Dotted lines; possible and/or occasional transmission events.