What causes influenza?

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The flu virus is made up of 3 main types (A, B and C) which vary in their severity. Flu strains are named after the place where they are first identified and are given a unique number. Flu viruses are known to change to prolong their survival. Populations do not have any resistance to new strains.

Types of Influenza

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Influenza A viruses are divided into subtypes and named according to two different proteins on their surface: haemagglutinin (HA) and neuraminidase (NA). For example, an “H5N1” virus has an HA 5 protein and an NA 1 protein. Many different combinations of HA and NA proteins are possible. Subtypes of influenza A that are currently circulating among people worldwide include H1N1, H1N2, and H3N2 viruses.


Influenza A

Influenza B

Influenza C

  • Most serious type of the virus

  • Less severe than type A

  • Mildest form of the virus

  • Outbreaks usually seen every 2-3 years

  • Tends to affect children who have never been exposed to the virus

  • Symptoms are similar to a common cold

  • Likely to mutate to produce a new strain

  • Causes smaller outbreaks of the virus

  • Severe symptoms with high fever

  • Immunity develops after being infected

  • Can cause death, especially in vulnerable patients

  • Rarely mutates so little risk of being infected more than once


How is human flu different from other flu viruses?

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Type A influenza viruses can infect several animals, including birds, pigs & horses. All known subtypes of the virus circulate among wild birds, which are considered the natural hosts for influenza A viruses and are known as ‘avian influenza viruses'. Avian influenza viruses (bird flu) are different from regular flu viruses because they only spread from bird to bird and occasionally bird to person, but not person to person.

Avian flu has mostly infected birds in Asia. Once a bird is infected, they become sick and often die. Some people who handle the birds, such as farmers, have caught avian flu from the animals.


So far, about 100 people who have been infected with bird flu and more than half have died from it. With the regular flu, most healthy people will recover after a week or two of feeling sick. They usually don't even need special medicine. The young and elderly are more vulnerable and should have a flu jab that offers protection against the virus.


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