What is Influenza?

Influenza is a highly contagious viral disease of the respiratory tract. The disease is caused by a group of closely related subtypes of virus, and different subtypes infect different animal species. It derives its public health significance from the rapidity by which epidemics evolve and spread amongst the community, and associated widespread morbidity and serious complications such as viral or bacterial pneumonia which may be fatal.

The influenza virus contains two surface proteins, haemagglutinin (H) and neuraminidase (N), which are involved in the infection of the host and production of new virus. The H protein is involved in attaching the virus to the cells it infects, while the N protein assists the virus in detaching from the cell in which it is produced. For influenza A, 16 distinct forms of H have been identified (designated H1 to H16) and 9 distinct forms of the N (designated N1 to N9).

Influenza A viruses have infected many different animals, including ducks, chickens, pigs, whales, horses, and seals. However, certain subtypes of influenza A virus are specific to certain species, except for birds, which are hosts to all known subtypes of influenza A. Subtypes that have caused widespread illness in people either in the past or currently are H3N2, H2N2, H1N1, and H1N2. H1N1 and H3N2 subtypes also have caused outbreaks in pigs, and H7N7 and H3N8 viruses have caused outbreaks in horses.

Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. For example, until 1998, only H1N1 viruses circulated widely in the U.S. pig population. However, in 1998, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs. Most recently, H3N8 viruses from horses have crossed over and caused outbreaks in dogs.

For the influenza B virus, only one H and one N have been identified.