What are the different types of flu?
Flu viruses come in three basic types: A, B and C. They may also be classified into flu subtypes.
Flu types A and B cause most cases of flu with the typical symptoms of aches, fatigue, coughs and fever. Type C usually causes less severe symptoms.
Flu viruses are always evolving, and different types are more common at different times.
Once you've had flu, the body builds up some defences called antibodies to help stop you catching it again. However, when faced with a different strain of flu or if the virus has evolved, those defences won’t be as effective.
This constantly changing flu landscape is also why it is necessary for the flu vaccine to be changed each year. It is also important for people who have the flu jab to have it each year, rather than just once as is the case with many other vaccines. Type A and B flu types are included in the flu vaccine.
Flu is more common in the winter months. Because seasons vary around the world, some countries' flu seasons differ from the UK flu peak.
Type A flu virus
Type A flu virus is constantly changing and is generally responsible for the large flu epidemics - when the disease spreads throughout one or more communities within a country. Flu is usually spread by people who are already infected, through coughs, sneezes and touching surfaces.
Type A flu or influenza A viruses are capable of infecting animals, including birds. Bird flu, also known as avian flu, can sometimes spread to humans. Birds cannot carry type B or C flu viruses.
Swine flu, responsible for the flu pandemic - when an epidemic spreads throughout the world - in 2009-10, is another type A flu virus. It was called swine flu because of similarities with flu viruses affecting pigs.
Type B flu virus
The type B flu is found only in humans. Type B flu may cause a less severe reaction than type A flu virus, but occasionally, type B flu can still be extremely harmful. Influenza type B viruses are not classified by subtype and although they can cause epidemics they do not cause pandemics.
Type C flu virus
Influenza C viruses are also found in people. They are, however, milder than either type A or B. People generally do not become very ill from influenza type C viruses. Type C flu viruses do not cause epidemics or pandemics.