From CDC: The Basics
There are two main types of influenza (flu) virus: A and B. These viruses are responsible for seasonal flu epidemics each year.
Influenza A viruses are divided into subtypes based on two proteins on their surface: hemagglutinin (H) and 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 found in people are influenza A (H1N1) and influenza A (H3N2).
In the spring of 2009 a new influenza A (H1N1) virus emerged. This virus was very different from regular human influenza A (H1N1), and it caused the first influenza pandemic in more than 40 years. The virus was originally referred to as "swine flu" because laboratory testing showed that many of its genes were very similar to influenza viruses that normally occur in pigs (swine) in North America. But further study has shown that this new virus is very different from what normally circulates in North American pigs. It has two genes from flu viruses that normally circulate in pigs in Europe and Asia as well as bird (avian) and human genes. Scientists call this a "quadruple reassortant" virus. The virus is now referred to as "2009 H1N1" and has mostly 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 different strains.
Influenza type C causes a mild respiratory illness and is not thought to cause epidemics.
How Flu Viruses Mutate
Influenza viruses can mutate into new strains in two different ways-antigenic drift and antigenic shift.
"Antigenic drift" refers to small changes in the virus that happen continually over time. Antigenic drift produces new virus strains that may not be recognized by the body's immune system. As a result, antibodies against older strains no longer recognize the "newer" virus, and reinfection can occur. This is one of the main reasons why people can get the flu more than once.
"Antigenic shift" refers to an abrupt, major change in the influenza A virus that produces new hemagglutinin and/or new hemagglutinin and neuraminidase surface proteins. Such changes produce a new influenza A subtype virus that is so different from the previous subtype that most people do not have immunity to it. Such a "shift" occurred in the spring of 2009, and caused the pandemic of that year.
While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.
Transmission of Influenza from Animals to People
Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses and seals. Influenza B viruses circulate widely only among humans.
Wild birds are the primary natural reservoir for all subtypes of influenza A viruses, and are thought to be the source of influenza A in all other animals as well. Most influenza viruses cause asymptomatic or mild infection in birds; however, the range of symptoms varies greatly depending on the strain of virus. Infection with certain avian influenza A viruses (for example, some strains of H5 and H7 viruses) can cause widespread disease and death among some species of wild and especially domestic birds such as chickens and turkeys.
While it is unusual for people to get influenza infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses have been reported.
Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs exhibit symptoms similar to humans, such as coughing, fever and runny nose. Because pigs are susceptible to avian, human and swine influenza viruses, they may potentially be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus. For example, if a pig were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could mix (reassort) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus as well. The resulting new virus would have surface proteins not previously seen in human influenza viruses. If this new virus were transmitted easily from person to person an influenza pandemic could result, as has occurred throughout the history of human health.