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Influenza is caused by a virus, which was first identified in 1933. There are two main types that cause infection, influenza A and influenza B (Influenza C is an uncommon type that infrequently causes infection). Influenza A is usually a more severe infection than influenza B. Influenza A subtypes exist which are named according to the haemagglutinin and neuraminidase antigens on the viral surface.  

A respiratory illness associated with infection by the influenza virus. Symptoms include headache, fever, cough, sore throat, aching muscles and joints. There is a wide spectrum of illness ranging from minor symptoms through to pneumonia and death.


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Nurse Consultant
Deputy Director of Infection Prevention and Control

Moseley Hall Hospital

Telephone: 0121 466 6550 

Influenza makes people feel worse than an ordinary cold. For most people influenza infection is just a nasty experience, but for some it can lead to illnesses that are more serious. The most common complications of influenza are bronchitis and secondary bacterial pneumonia. These illnesses may require treatment in hospital and can be life threatening especially in the elderly, asthmatics and those in poor health. The influenza virus does not necessarily cause high mortality, but for old sick people it may speed up their death. During a pandemic, though, influenza can cause serious illness in young healthy individuals.

The most common symptoms of influenza are an abrupt onset of fever, shivering, headache, muscle ache and dry cough. Most people confuse influenza with a heavy cold; however influenza is usually a more severe illness than the common cold, which is caused by other respiratory viruses such as rhinovirus.

Cold symptoms are limited to the upper respiratory tract with runny nose, sneezing, watery eyes and throat irritation. The symptoms usually occur gradually and do not cause a fever or body aches.

Influenza occurs most often in the winter months and usually peaks between December and March in the northern hemisphere. Illnesses resembling influenza may occur in the summer months but they are usually due to other viruses.  The timing of influenza activity is unpredictable.

In temperate climates influenza tends to strikes from late autumn through to spring, although technically influenza is not bound by seasons, and can occur all year round in tropical climates. A possible explanation for the high influenza virus activity in the wintertime is that people congregating indoors during winter facilitate the transmission of the virus or that more humid air indoors may help the viruses survive longer.

Usually, a doctor will diagnose a case of the flu based on typical symptoms of fever, chills, headache, cough and body aches. Specific lab tests to confirm the flu are costly and time consuming and are usually limited to outbreak or disease surveillance efforts.

The flu virus is highly contagious and is easily passed from person-to-person when an infected person coughs or sneezes. Transmission can also occur by touching a surface contaminated with respiratory secretions and then putting the fingers in the mouth or nose or near the eyes. The flu virus can live on a hard surface for up to 24 hours and a soft surface for around 20 minutes.

To reduce transmission, it is vital that if someone has a respiratory infection that they cover their nose and mouth when they cough and sneeze, preferably with a tissue, and wash their hands afterwards.  Tissues need to be bagged and disposed of appropriately if they are used outside the home; otherwise they can be disposed of in normal household waste.

Normal household products can be used to clean the room of someone who has had flu as the virus can easily be destroyed. Open the windows, wash bedlinen but make sure you wash your hands afterwards, pay particular attention to hard surfaces and allow as much contact time with the cleaning product before wiping it clean.

The incubation period – the period between infection and the appearance of symptoms - is about two to three days. Although virus has been detected before symptoms appear, adults are usually considered infectious once symptoms appear and for 3-5 days afterwards. This period is longer in children.

Rest, drink plenty of fluids and take analgesics (paracetamol for all ages, aspirin may be taken by adults).

Most influenza-like illnesses are self-limiting and may be caused either by influenza or other viruses/pathogens. It is best to treat the infection at home until the person is well enough to return to normal activities. Medical advice should be sought if symptoms become severe or last more than about a week. Those with chronic or long-standing illness may need medical attention earlier.

Who is most at risk from the complications of influenza?

The young have a greater risk of being infected because they have not developed immunity to the virus.

The elderly have a greater risk of the severe complications of infection such as pneumonia, because they often have underlying diseases, which reduce their resistance to infection. The immune response may also be less effective in elderly persons.

The high-risk groups include individuals whose respiratory, cardiac or immune systems make them more vulnerable to flu and more likely to suffer severe illness.

Routine vaccination offers the best protection and people who are at high risk of infection should be vaccinated. It is difficult to avoid infection if there is an epidemic. Keeping away from crowded places can reduce the risk of becoming infected and spreading it to others. A previous flu infection or vaccination will not necessarily provide protection against further infections because the virus is continually changing genetically and different subtypes circulate each winter. Please talk to your GP if you have any questions about flu vaccination.