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Learn About Different Types of Flu

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Updated March 29, 2010

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Did you know there are several different types of flu? Even when it comes to seasonal flu, there are three different types - two of which cause serious illness in humans. Learn about all the types of flu, how they are classified and what they mean to you.

Seasonal Flu

Seasonal flu is the type of flu that typically causes illness for just a few months out of the year. Flu season is different depending on where you are in the world. In the United States, it usually occurs sometime between October and April. There are three types of flu viruses that cause seasonal influenza. They are classified as influenza A, B and C.

Influenza A

Influenza A usually causes a majority of seasonal flu. There are many different varieties of influenza A which are further classified into subtypes (H and N) and even further into different strains.

H and N subtypes

H and N subtypes of the influenza virus are based on the proteins that are attached to the virus. There are 16 different types of hemagglutinin (H) proteins and 9 different types of neuraminidase (N). This is how gets names such as H1N1 or H3N2. However, the pandemic H1N1 influenza is different because it was created from a combination of human, swine and bird flu viruses. So although it is an influenza A virus, it is a mutation and is not the same as the influenza A that causes seasonal flu.

Influenza B

Influenza B is another type of flu that causes seasonal illness. There are different strains of influenza B, but it is not subtyped like influenza A is.

Influenza C

Influenza C causes only mild respiratory illnesses and is not known to have caused any seasonal flu epidemics. Most people who get influenza C will not know that they have a "flu" virus because symptoms are similar to those of a cold.

Flu Pandemic

Any flu virus can become a flu pandemic, during which there are mass outbreaks of illness in humans around the world in a relatively short amount of time. Some flu pandemics in the past have caused very severe illness and killed millions of people (such as the 1918 flu pandemic) and others are less serious.

H1N1 (Swine Flu)

In the spring of 2009, a new influenza A virus was discovered in Mexico and quickly spread throughout the United States, North America and around the world. It was determined to be a combination of human, swine and bird flu and became the first flu pandemic the world had seen in over 40 years.

Bird Flu

H5N1 is the strain of influenza that is known as bird or avian flu. It is usually only transmitted between birds but can be passed from bird to human. It is associated with very serious illness, multi-organ failure and high death rates when it does spread to humans. There is some fear that if it mutates and becomes a virus that spread easily from person to person, that it could cause a very serious flu pandemic.

Sources:

"Types of Influenza Viruses." Seasonal Flu 26 Aug 09. Centers for Disease Control and Prevention. 28 Mar 10.

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