Understanding the 3 Different Types of Flu

The ABCs of Flu Viruses

Three generation family sitting sick in sofa
The different kinds of flu. Marie-Reine Mattera / Getty Images

Did you know that there are several different types of influenza? 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 falls between October and April. There are three types of flu viruses that cause seasonal influenza: A, B, and C.

Influenza A

Type A flu is usually responsible for the majority of seasonal flu cases. It is found in humans and in animals. Influenza A is spread from person to person by people who are already infected. Touching objects the infected person has touched (doorknobs, faucets, phones) or even being in the same room as the person, especially if they are coughing or sneezing, is enough to become infected yourself. There are many different varieties of influenza A that are classified into subtypes - H and N - and even further into different strains.

H & N Subtypes

H and N subtypes of influenza A are based on the particular proteins that are attached to the virus. There are 16 different types of hemagglutinin (H) proteins and nine different types of neuraminidase (N) proteins.

This is how names such as "H1N1" or "H3N2" are acquired.

However, the pandemic H1N1 influenza is different because it was created from a combination of human, swine and bird flu viruses. Although it is technically an influenza A virus, it is a mutation and therefore not the same as the influenza A that causes seasonal flu.

Influenza B

Type B flu is another type of flu that causes seasonal illness. It is found only in humans. Influenza B has the potential to be very dangerous, but it is typically less severe than influenza A. It does not cause pandemics. Influenza B is typically less severe than influenza A, but it can still be dangerous. There are different strains of influenza B, but they are not sub-typed.

Influenza C

Type C flu, which affects only humans, is much milder than types A and B. It typically causes mild respiratory illnesses and it is not known to have caused any seasonal flu epidemics. Most people who contract influenza C will know that they have some strain of the flu virus because symptoms are similar to those of a cold.

Flu Pandemic

Any flu virus has the potential to become a flu pandemic, during which there are mass outbreaks of illness in humans around the world in a relatively short amount of time. In the past, some flu pandemics have caused very severe illness and killed millions of people, such as the 1918 flu pandemic.

Others are less serious.

H1N1 - Swine Flu

In the spring of 2009, a new influenza A virus was discovered in Mexico: H1N1, or swine flu. It quickly spread throughout North America, the United States and around the world. H1N1 is a combination of human, swine and bird flu. It became the first flu pandemic the world had seen in more than 40 years.

H5N1 - Bird Flu

H5N1 is the strain of influenza known as bird or avian flu. Typically it is transmitted between birds, but it can be passed from bird to human. It does not spread from person to person. When it does infect humans, bird flu is associated with very serious illness, multi-organ failure, and high death rates. In fact, bird flu has killed more than half of people infected with it.

Although your risk of contracting bird flu is very low, there is some grave fear surrounding H5N1. If the strain mutates and becomes a virus that spreads easily from person to person, it could cause a very serious flu pandemic.


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

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