What Is Influenza A?

Graphic representation of an influenza virus.. Image courtesy: US Centers for Disease Control and Prevention (CDC)

Influenza, or the flu, comes in hundreds of different strains. The virus mutates frequently, but the strains are classified into one of three main categories - A, B or C. Influenza A is the group that most commonly causes illness in humans.

Further Classification

All influenza A viruses are further broken down into H and N subtypes. So, any influenza virus that is described as "H#N#" (such as H1N1) is an influenza A virus.

There are 16 H subtypes and nine N subtypes, but only three combinations have actually caused highly contagious illness in humans. Other combinations have been found to infect other species (such as birds and pigs), but they have not caused widespread human infections. The three combinations that cause almost all outbreaks of the flu in humans are H1N1, H2N2 and H3N2.

Even in these subtypes, the influenza virus can mutate and change each year. For this reason, influenza viruses are also named using:

  • The host of origin (swine, chicken, etc., or no host if it is of human origin)
  • The geographical location of origin (Hong Kong, Alberta, etc.)
  • Strain number
  • Year of discovery (or isolation)

When the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) name a new strain of the influenza virus, they start with the group (A, B or C), then list the host, location of origin, strain number, year of discovery and H-N subtype in parentheses.

An example of an influenza A virus would look like this:

  • A/duck/Alberta/35/76 (H1N1) for a virus from duck origin

History of Influenza A

All of the major flu pandemics in modern history have been caused by influenza A viruses. The 1918 pandemic - also known as the Spanish flu - was caused by an H1N1 virus.

The 1957 flu pandemic - also called the Asian flu - was caused by an H2N2 virus. The 1968 pandemic - also called the Hong Kong flu - was caused by an H3N2 virus. Finally, the 2009 pandemic - called swine flu - was caused by a novel H1N1 virus.

Influenza A in Flu Vaccines

The seasonal flu vaccine typically contains two different strains of influenza A and one strain of influenza B. The strains included in the vaccine are the same for all types of flu vaccines in a given year, but may change from year to year.

Influenza A and You

When you get sick with the flu, you will have such symptoms as coughing, fever, body aches, exhaustion and headache. Not all strains or subtypes of influenza A will cause these symptoms, but this is generally how our bodies react to the influenza A viruses that cause the terrible illness we know as the flu.

If you experience flu symptoms, your healthcare provider may be able to perform a flu test to determine whether or not your illness is caused by influenza.

Some of these tests can even differentiate between influenza A and B.

If you are diagnosed with the flu, your treatment will vary depending on your health history, the severity of your symptoms and at what point in your illness you are diagnosed. Your healthcare provider will help determine what kind of treatment is right for you.


Influenza (Flu) Research. Seasonal Influenza (Flu) 08 Feb 11. US Centers for Disease Control and Prevention. 21 Jan 12.

Types of Influenza Viruses. Seasonal Influenza (Flu) 10 Nov 11. US Centers for Disease Control and Prevention. 21 Jan 12.

History of Flu Pandemics. Pandemic Awareness. Flu.gov. 21 Jan 12.

Dynasty: Influenza Virus in 1918 and Today. NIH News 29 June 09. National Institute of Allergy and Infectious Diseases. National Institutes of Health. 21 Jan 12.

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