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:

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.

This genus has one species, influenza C virus, which infects humans, dogs and pigs, sometimes causing both severe illness and local epidemics. However, influenza C is less common than the other types and usually only causes mild disease in children. Flu due to the Type C species is rare compared to Types A or B, but can be severe and can cause local epidemics. Type C has 7 RNA segments and encodes 9 proteins, while Types A and B have 8 RNA segments and encode at least 10 proteins.

Because influenza virus A has an animal reservoir that contains all the known subtypes and can undergo antigenic shift, this type of influenza virus is capable of producing pandemics. Influenza viruses A and B also cause seasonal epidemics every year due to their ability to antigenic shift. Influenza virus C does not have this capability and it is not thought to be a significant concern for human health. Therefore, there are no vaccinations against influenza virus C.

Influenza D virus is a species in the virus genus Influenzavirus D, in the family Orthomyxoviridae, that causes influenza. Influenza D viruses are known to infect pigs and cattle; no human infections from this virus have been observed. First isolated from pigs in 2011, the virus was categorized as a new genus of Orthomyxoviridae in 2016, distinct from the previously-known Influenzavirus C genus; before then, Influenza D virus was thought to be a subtype of Influenzavirus C. Cases of infections from the Type D virus are rare compared to Types A, B, and C. Similar to Type C, Type D has 7 RNA segments and encodes 9 proteins, while Types A and B have 8 RNA segments and encode at least 10 proteins.

Because influenza virus A has an animal reservoir that contains all the known subtypes and can undergo antigenic shift, this type of influenza virus is capable of producing pandemics. Influenza viruses A and B also cause seasonal epidemics every year due to their ability to antigenic shift. Influenza viruses C and D do not have this capability, and they have not been implicated in any pandemics; thus, there are currently no human vaccines available for Influenza viruses C or D. An inactivated Influenzavirus D vaccine was developed for cattle; however, the vaccine only provided partial protection in challenge experiments.