Commonly known as the flu, influenza is a viral infection of the upper airways. It usually occurs mainly in winter (seasonal flu), but may occur at any time. Outbreaks of particular flu variants, such as swine flu and bird flu, may also occur. The virus is typically spread by droplets from coughs and sneezes. Infection can occur by inhaling the droplets or by touching contaminated surfaces and then touching the mouth or nose.
Seasonal flu and swine flu are spread in these ways. Bird flu is usually caught directly from infected birds.
All types of flu produce similar symptoms, including a fever, headache, coughing or sneezing, exhaustion, aches, a stuffy or runny nose, vomiting, and diarrhea.
Flu symptoms usually clear up within a few days without treatment, although tiredness may persist.
Occasionally, the condition can cause serious illness or complications such as pneumonia, and those at particular risk may sometimes be offered antiviral medication.
A flu vaccine is available to reduce the risk of getting the flu. The flu virus changes (mutates) frequently, so it is necessary to be vaccinated each year against the strains of virus prevalent at the time.

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