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Influenza viruses are categorized as three types: A, B and C. The seasonal flu that we get every year is caused by the human influenza A and B viruses.
Influenza C virus causes only mild respiratory illnesses and are not dangerous enough to cause an epidemic.

According to the CDC, Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus has caused an influenza pandemic.

Influenza B viruses are not divided into subtypes. Influenza B viruses also can be further broken down into different strains.

Regular influenza A (H1N1), A (H3N2), and influenza B viruses are included in each year's seasonal influenza vaccine. Getting a flu vaccine can protect against flu viruses that are the same or related to the viruses in the vaccine. The seasonal flu vaccine does not protect against influenza C viruses and this year’s seasonal vaccine will not protect against the 2009 H1N1 virus.