skip to the content
About the flu

Types of flu

Different types of flu virus

Flu is caused by a virus. The virus mainly attacks the respiratory tract – that is, the nose, sinuses, throat, lungs and middle ear. However, flu can also cause symptoms and complications in other parts of the body.
The flu virus was first isolated in the 1930s, although flu as a disease has probably existed for centuries. Three different flu viruses cause disease in humans – these are known as types A, B and C. The different flu viruses vary in terms of how easily they can be passed from person-to-person as well as in terms of the severity of illness they cause.  Influenza A and B are the most important in terms of causing illness in humans.

  • Influenza A is the most common type of flu virus – it is also the type of flu virus most likely to cause a serious epidemic or pandemic. [1]
  • Influenza B can cause epidemics. [1] While it is clinically indistinguishable from influenza A, it tends to occur mostly in younger age groups. [2]
  • Influenza C has never been connected with a large epidemic and usually just causes mild respiratory infections similar to the common cold. [3]

Bird flu

Bird flu – also known as avian influenza – is caused by type A flu viruses that occur naturally among wild birds. Most bird flu viruses do not cause disease in humans.

To view a short film about the flu virus and how it infects cells, follow the link.

To view a short film about how flu viruses change each year, follow the link.

  1. Laver WG, Bischofberger N, Webster RG. Disarming flu viruses. Sci Am 1999; 280: 78–87.
  2. Nicholson K. Human influenza. In: Nicholson K, Webster RG, Hay A, eds. Textbook of influenza. Oxford, UK: Blackwell Science, 1998: 219–64.
  3. Matsuzaki Y, Katsushima N, Nagai Y, et al. Clinical features of influenza C virus infection in children. J Infect Dis 2006; 193: 1229–35.
  • "I would describe flu as something that makes you feel very, very ill. You get a headache, aching bones, and are generally fed up"
  • "When I get flu symptoms I feel like a train has run over me"
  • "I felt very sick and, during the first week, I had high temperature. Flu was very different to a cold. I went back to work after two and a half weeks. Then I suffered a setback for another week"
  • "I run a guesthouse; flu would be a real pest for me. I couldn’t cook, I shouldn’t cook, and I wouldn’t cook, so I would need to get a lot of extra help in. It would cost me a lot more money. So flu’s a real problem, a right downturn"
  • "I work on my own and when I can’t work, I have to try not to infect my little children, so they avoid getting the flu as well"
  • "I feel a little numb, like I’m in another dimension. Because in general I have sore throat, headache, I feel all clogged-up. Usually a sensation like I’m floating in the air"
  • "Shivers, sweats, makes you ache. Last time I had flu I was off for two weeks"
  • "I think that I would probably be concerned that I would pass flu onto my children, who might not be able to deal with it as well as I would"
  • "Flu makes you feel like you’ve been hit over the head with a baseball bat. You don’t feel like you can get yourself out of bed as all your energy is drawn from you"
  • "I was incapable of working. It just wasn’t possible at all… and my flu dragged on for more than a week"
  • "I have no strength. It annoys me because I can do nothing, it seems like I’m wasting my time"
  • "Flu’s like being hit by a truck because you feel completely gone. There is no energy left in your body. You are suffering from high temperature. You’re sweating a lot and you feel really unwell"
  • "First of all infinite tiredness, then I feel like sleeping and don’t want to eat"
  • "I felt really miserable because my muscles and my bones were aching. Well, I can hardly describe it, I just felt really awful, absolutely miserable"
  • "It’s like there’s something huge treading on your head, like a deafening noise that destroys the eardrums"
  • "The fever was constantly rising – I immediately measured my temperature as soon as I got home. It was 41°C and it kept rising and I was wondering what’s going on. I was really scared! Everything was hurting and it all happened so fast. Flu came out of the blue"
  • "I always feel like my limbs have iron weights tied to them and I’m going to fall over any minute, a really stuffy runny nose and feel antisocial"
  • "I was totally dependent on others! I was incapable of doing anything at all"
  • "The real flu knocks you for six – you just don’t want to move or do anything. It’s not very nice"
  • "Not being able to go to work. Not being able to do the things I have to do at home and the commitments that I have day by day"