In virus classification influenza viruses are RNA viruses that make up three of the five genera of the family Orthomyxoviridae:
These viruses are only distantly related to the human parainfluenza viruses, which are RNA viruses belonging to the paramyxovirus family that are a common cause of respiratory infections in children such as croup, but can also cause a disease similar to influenza in adults.
Figure 1. Noimanclature of influenza viruses
This genus has one species, influenza A virus. Wild aquatic birds are the natural hosts for a large variety of influenza A. Occasionally, viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics. The type A viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses. The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:
|H1N1,||which caused Spanish Flu in 1918, and Swine Flu in 2009|
|H1N2,||endemic in humans, pigs and birds|
|H2N2,||which caused Asian Flu in 1957|
|H3N2,||which caused Hong Kong Flu in 1968|
|H5N1,||which caused Bird Flu in 2004|
|H7N7,||which has unusual zoonotic potential|
This genus has one species, influenza B virus. Influenza B almost exclusively infects humans and is less common than influenza A. The only other animals known to be susceptible to influenza B infection are the seal and the ferret. This type of influenza mutates at a rate 2–3 times slower than type A and consequently is less genetically diverse, with only one influenza B serotype. As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible. This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.