Many viral illnesses are casually called "the flu." However, true influenza is caused by specific viruses in the family Orthomyxoviridae. There are many forms of the influenza virus, which are categorized by types, subtypes and strains. Only influenza A viruses have subtypes.
Types, Subtypes and Strains
Three types of influenza viruses exist in the Orthomyxoviridae family--A, B and C. All three types infect humans, however only types A and B cause seasonal outbreaks of flu. Influenza B viruses are categorized by strains whereas influenza A viruses are categorized by both subtype and strain.
The categorization of influenza A is so complicated because influenza A viruses change rapidly. Accurately tracking these changes requires a three-tiered hierarchy: type, subtype and strain. The important point to remember is that subtypes apply only to influenza A viruses.
Subtypes Defined
Influenza viruses have an outer coat that contains two surface proteins. Variants of these surface proteins--hemagglutinin and neuraminidase--determine the influenza A subtype. There are 16 hemagglutinin subtypes (H1 to H16) and nine neuraminidase subtypes (N1 to N9). Influenza A subtypes are named according to the combination of their hemagglutinin and neuraminidase proteins. For example, influenza A subtype H1N1 is so named because it has proteins H1 and N1 on its surface.
Evolution of Subtypes
Influenza A viruses are constantly evolving due to frequent, spontaneous changes in their genetic makeup. These changes are called mutations. Typically, mutations cause relatively minor changes in the H and N surface proteins. These minor changes are known as antigenic drift, a perpetual process with influenza A viruses.
While antigenic drift is constantly taking place, antigenic shifts occur only occasionally. Antigenic shifts result from mutations that lead to the formation of a new subtype that bears little resemblance to its immediate ancestors. According to the U.S. Centers for Disease Control and Prevention, antigenic shifts are important because they may signal the onset of a new influenza pandemic.
It's helpful to think of antigenic drift and shift in terms of color. Say the current influenza A subtype has a cherry red H protein. Over time, minor mutations cause the color of the H protein to change from cherry red to crimson to ruby red. This is akin to antigenic drift--the H protein is still red, but the shade of red is drifting over time. Now imagine that a major mutation occurs that results in the H protein changing from red to green. This major change would be akin to an antigenic shift.
Subtypes and Hosts
Wild birds are the natural hosts of influenza A viruses. However, these viruses infect other animals including pigs, horses, and of course, humans. While all known influenza A subtypes infect birds, only a few subtypes infect humans.
Concern among public health agencies was raised beginning in 2003 when certain subtypes of influenza A that typically infect only birds were reported to rarely jump species and infect humans, causing severe illness and an unusually high death rate. These avian influenza A subtypes, commonly known as avian or bird flu, are now closely monitored to watch for mutations that may make them more infective to humans.
History in Modern Times
The U.S. Department of Health and Human Services reports that "influenza pandemics have probably happened during at least the last four centuries." Since acquiring the scientific knowledge to track the evolution of influenza A subtypes in the early 20th century, four flu pandemics have occurred, all caused by influenza A viruses. The pandemics of 1918 and 2009 (Spanish and swine flu) were caused by influenza A subtype H1N1, and those in 1957 and 1968 (Asian flu and Hong Kong flu) were due to influenza A subtype H2N2 viruses.
