What Are The Different Kinds Of Flu?

Several different yet closely related viruses cause influenza, or the flu. Three influenza virus groups---types A, B and C---belong to the Orthomyxoviridae family of viruses, which includes a variety of viruses that infect birds and mammals. Within each influenza type, different strains exist. When doctors and scientists talk about different kinds of flu, they are referring to the various types and strains of human influenza viruses.

Influenza Type A

Influenza type A viruses are one of the two types of influenza viruses responsible for seasonal influenza, notes the World Health Organization. Influenza A viruses undergo frequent genetic changes, or mutations, that cause the viruses to vary from one flu season to the next. Periodically, a dramatic genetic shift occurs, generating what scientists term a "novel" virus. This occurred in the 2009-2010 flu season with the emergence of the novel H1N1 influenza type A virus.
The emergence of a novel influenza virus proves significant because it can lead to a flu pandemic---infections with the "new" virus occurring throughout most of the world. Importantly, the word "pandemic" refers only to the global distribution of the virus, not to the severity of the associated illness. The medical reference text "Principles and Practice of Infectious Diseases" explains that only influenza A type viruses undergo the dramatic genetic shifts that lead to the emergence of a novel influenza virus. Therefore, only influenza type A viruses can cause flu pandemics.

Influenza Type B

Influenza type B viruses, along with type A viruses, cause seasonal influenza. Influenza type B viruses undergo genetic changes; however, they do so more slowly than do type A viruses. The Centers for Disease Control and Prevention points out that although influenza type B viruses can cause flu epidemics with large numbers of people infected in a specific geographic location, they do not cause pandemics. Dr. Margaret Hunt of the University of South Carolina School of Medicine notes in the electronic text "Microbiology and Immunology Online" that overall, symptoms associated with influenza type B infections are milder than those associated with influenza type A infections. Seasonal influenza vaccines provide protection against infection with circulating variants of both influenza types A and B.

Influenza Type C

The World Health Organization reports that influenza type C viruses cause far fewer cases of the flu than type A and type B viruses. In general, influenza type C viruses cause a milder form of the flu compared with influenza types A and B. The CDC notes that influenza type C does not cause epidemics or pandemics. Due to the low frequency of influenza type C infections and the mild illness associated with infection, seasonal influenza vaccines do not include protection against this type of influenza.