What is influenza?

Most people have experienced an influenza-like illness, with symptoms including: fever accompanied by respiratory symptoms such as sneezing, coughing, runny nose and sore throat and systemic symptoms such as muscle aches, chills and headaches. Gastrointestinal symptoms such as nausea, vomiting and diarrhoea are also common. Flu viruses on a map Copyrighted image Credit: Map: Roberto1977 Flu virus: Axel Kock | Dreamstime.com Help us track the spread of flu viruses

Influenza is caused by two types of virus (influenza A or influenza B), which are highly contagious and are spread from person to person by droplets of respiratory secretions produced by sneezing and coughing. Infection is usually self-limiting and lasts for three to four days, with some symptoms persisting for one to two weeks.

Several different infectious agents can cause flu-like symptoms, and further laboratory testing is required before infection with influenza A or B virus can be confirmed.

It is estimated that yearly influenza epidemics in the UK cause between 12,000 and 13,800 deaths.

How does influenza spread?

In the Northern hemisphere, influenza occurs in a seasonal pattern with epidemics in the winter months, typically between December and March. Every ten years or so, a different strain of virus spreads through the global population (for example, 'swine flu'), causing a pandemic.

Influenza activity is monitored through surveillance schemes, which record the number of new GP consultations for influenza-like illness per week per 100,000 population. In England, normal seasonal activity is currently defined as 30–200 consultations, with greater than 200 defined as an epidemic.

You can view current clinical data on influenza-like illnesses in the UK on the Royal College of GPs website

In the USA, the data is available from the Centers For Disease Control and Prevention.

How you can help us

Currently, a great deal of information about influenza-like illnesses is not recorded, because many people treat themselves at home. It is only when cases become serious enough to be reported to a doctor that official statistics become available.

To capture some of this 'missing' data, we would like you to use our flu mapping tool.

Further learning

This tool builds on learning resources about influenza and epidemiology that are part of the Open University module Infectious disease and public health.

Try a free course and explore the biology of influenza as well as a range of topics including: the virus, infection, replication, mutation, immune responses, pathology, surveillance, diagnosis and treatment.

You might also find this article useful where we draw together our latest germ-filled videos and further learning links.

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