Human epidemic influenza is caused by influenza type A and B viruses, which continually undergo antigenic change in their surface antigens, haemagglutinin (H) and neuraminidase (N). Influenza epidemics are the consequence of small, ongoing antigenic changes known as "antigenic drift", which occurs in both influenza types. Pandemic influenza occurs at irregular and unpredictable intervals, and is the result of a major antigenic change known as "antigenic shift", which occurs only in influenza A. Aquatic birds are the evolutionary hosts of influenza viruses; they harbour many distinct forms or subtypes of influenza A, which are usually present as harmless gut infections. Antigenic shift involves the evolution of a new human influenza A virus through the acquisition of a new haemagglutinin gene encoding a different subtype from an avian influenza, or by the adaptation of an avian virus, causing it to become transmissible between humans. Two subtypes of avian influenza, H5 and H7, can cause severe infections when introduced into domestic poultry. Recently, influenza A/H5N1 viruses have caused widespread outbreaks, starting in Asia and spreading widely to other regions. Avian influenza viruses do not readily infect humans. However, during the past 3 years, more than 250 cases of H5N1 infection of humans have occurred, with associated mortality approaching 60%. It is feared that a new pandemic of human influenza may emerge from this.