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About Influenza

Types of influenza

Epidemics

Seasonal – or interpandemic – influenza tends to be more pronounced in the colder months of the year. In temperate zones the influenza season typically starts in autumn and lasts through to winter – from October through to April in the northern hemisphere and May through to September in the southern hemisphere. [1] Although seasonality is more difficult to detect in tropical zones, influenza activity is more likely during monsoon and wet seasons.

The reasons behind the seasonal pattern are unclear, but may be due to:

  • prolonged periods spent indoors and in close contact with others during periods of wet and/or cold weather
  • increased viability of the influenza virus (A and B) on exposed surfaces (doorknobs, countertops, etc.) in low humidity and low temperature conditions
  • cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles
  • effect of vitamin D levels on immunity to the virus [2]; vitamin D levels fall when people stay out of the sun. 

During an influenza epidemic there is an upsurge of influenza activity that lasts for 5–6 weeks before declining sharply. [1]
More information about the impact of epidemic influenza, can be found here.

Pandemics

Global outbreaks of influenza  – pandemics – occur when the influenza virus mutates to create a new antigenic subtype. This is expected to happen every 10–40 years. [3] Since humans have little or no immunity to this new virus type, an influenza pandemic can affect up to 50% of the world's population. [3]

Experts believe that the next influenza pandemic could quite possibly result from a mutation in the H5N1 avian influenza virus. Such a mutation will allow the virus to infect humans more readily and be transmitted easily from person-to-person.

More information on pandemics and avian influenza – such as where in the world avian influenza has been detected, the potential impact of another global outbreak, and the worldwide pandemic preparedness plans – can be found here.

  1. Nicholson K. Human influenza. In: Nicholson K, Webster RG, Hay A, eds. Textbook of influenza. Oxford, UK: Blackwell Science, 1998: 219–64.
  2. Cannell JJ, Vieth R, Umhau JC, et al. Epidemic influenza and vitamin D. Epidemiol Infect 2006; 134: 1129–40.
  3. Oxford JS, Lambkin R. Targeting influenza virus neuraminidase – a new strategy for antiviral therapy. Drug Discovery Today 1998; 3: 448–56.
  • "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"