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Influenza Viruses

Types, Subtypes, and Strains

There are three types of influenza viruses: A, B, and C. Combined they are responsible for producing 306 human influenza viruses. Only influenza A viruses are further classified by subtype on the basis of the two main surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). Influenza A subtypes and B viruses are further classified by strains.

Human Influenza Viruses and Avian Influenza A Viruses

Humans can be infected with influenza types A, B, and C viruses. Subtypes of influenza A that are currently circulating among people worldwide include H1N1 (Swine Flu), H1N2, and H3N2 viruses.

Wild birds are the natural host for all known subtypes of influenza A viruses. Typically, wild birds do not become sick when they are infected with avian influenza A viruses. However, domestic poultry and swine can become very sick and die from avian influenza.

How Influenza Viruses Change: Drift and Shift

Influenza viruses are dynamic and are continuously evolving. Influenza viruses can change in two ways: antigenic drift and antigenic shift. Influenza viruses are changing by antigenic shift all the time, but antigenic drift happens only occasionally. Influenza type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift. Antigenic drift refers to small, gradual changes that occur through point mutations in the two genes that contain the genetic material to produce the main surface proteins, hemagglutinin, and neuraminidase. These point mutations occur unpredictably and result in minor changes to these surface proteins. Antigenic shift produces new viral strains through genetic reassortment. These new viruses may not be recognized by antibodies to earlier influenza strains, leaving the population vulnerable to a new viral outbreak. 

Influenza Type A and Its Subtypes

Influenza type A viruses are divided into subtypes and named on the basis of two proteins on the surface of the virus. There are 16 known HA subtypes and 9 known NA subtypes. Many different combinations of HA and NA proteins are possible. Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in general circulation among people including the 2009 Swine Flu outbreak. Only influenza A viruses infect birds and all known subtypes of influenza A viruses can infect birds. However, there are substantial genetic differences between the influenza A subtypes that typically infect birds and those that infect both humans and birds. Three prominent subtypes of the avian influenza A viruses that are known to infect both birds and humans are:

Influenza A H5

Nine potential subtypes of H5 are known. H5 infections, such as HPAI H5N1 viruses currently circulating in Asia and Europe, have been documented among humans and sometimes cause severe illness or death.

Influenza A H7

Ten potential subtypes of H7 are known. H7 infection in humans is rare but can occur among persons who have direct contact with infected birds. Symptoms may include conjunctivitis and/or upper respiratory symptoms. H7 viruses have been associated with both Low Pathogenic Avian Influenza: LPAI (e.g., H7N2, H7N7) and High Pathogenic Avian Influenza: HPAI (e.g., H7N3, H7N7, H7N9), and have caused mild to severe and fatal illness in humans. Currently, there is no evidence that H7 can spread from human to human.

Influenza A H9

Nine potential subtypes of H9 are known; influenza A H9 has rarely been reported to infect humans. This subtype has been documented only in a low pathogenic form.

Influenza Type B

Influenza B viruses are usually found only in humans. Unlike influenza A viruses, these viruses are not classified according to subtype. Influenza B viruses can cause morbidity and mortality among humans, but in general are associated with less severe epidemics than influenza A viruses. Although influenza type B viruses can cause human epidemics, they have not caused pandemics.

Influenza Type C

Influenza type C viruses cause mild illness in humans and do not cause epidemics or pandemics. These viruses are not classified according to subtype.


Influenza B viruses and subtypes of influenza A virus are further characterized into strains. There are many different strains of influenza B viruses and of influenza A subtypes. New strains of influenza viruses appear and replace older strains. This process occurs through antigenic drift. When a new strain of human influenza virus emerges, antibody protection that may have been developed in humans after infection or vaccination with an older strain may not provide protection against the new strain.


Enteroviruses are small viruses made of ribonucleic acid (RNA) and protein. This group includes the polioviruses, coxsackieviruses and echoviruses. There are 62 non-polio enteroviruses that can cause disease in humans: 23 Coxsackie A viruses, 6 Coxsackie B viruses and 28 echoviruses and 5 other enteroviruses.

Non-Polio Enteroviruses

Non-polio enteroviruses are very common and second only to the “common cold” virus, the rhinovirus, as the most common viral infectious agent in humans. Everyone is at risk of infection if they do not have immunity to a specific enterovirus. Enterovirus outbreaks commonly occur during the Summer through Fall.