The Influenza Virus
There are three types of influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics. The emergence of a new and very different influenza virus to infect people can cause an influenza pandemic. Influenza type
C infections cause a mild respiratory illness and are not thought to cause epidemics.
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 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes. (H1 through H18 and N1 through N11 respectively.)
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 the human influenza A (H1N1) viruses circulating at that
time. The new virus caused the first influenza pandemic in more than 40 years. That virus (often called “2009 H1N1”) has now
replaced the H1N1 virus that was previously circulating in humans.
Influenza B viruses are not divided into subtypes, but can be further broken down into lineages and strains. Currently
circulating influenza B viruses belong to one of two lineages: B/Yamagata and B/Victoria.