Influenza (flu)

Key disease facts

Influenza (flu) is a very common, highly infectious disease caused by a virus. It can be very dangerous, causing serious complications and death, especially for people in risk groups. In rare cases flu can kill people who are otherwise healthy. In the UK an average of 600 people a year die from complications of flu, but in some years this can rise to over 10,000 people. Flu leads to hundreds of thousands of GP visits and tens of thousands of hospital stays a year.

The flu virus is very variable and changes over time. Each year there are different strains around, and a new vaccine has to be prepared to deal with them. Vaccination from previous years is not likely to protect people against current strains of flu.

There are three basic types of flu: A, B and C. Type A is the most dangerous; it is the one that can cause serious disease and also triggers worldwide pandemics. Type C causes mild disease. Type B can make you feel very ill, but it has never led to a worldwide pandemic.

Flu epidemics can kill thousands or even millions of people. The 1918 flu epidemic is estimated to have affected half the world's population, and killed 40-50 million people worldwide. The World Health Organization (WHO) estimates that flu kills between 250,000 and 500,000 people around the world every year.

In the UK (and in the rest of the northern hemisphere) the annual flu season runs from about October to March or April. Most cases of flu occur between December and February.

What are the symptoms?

The most common symptoms of flu are:

  • sudden high temperature of 38°C or above
  • severe headache
  • general aches, pains, tiredness and weakness
  • shivering and chills
  • aching muscles, pain in limbs or joints
  • sore throat
  • a runny or blocked nose, and sneezing
  • a dry, chesty cough
  • difficulty sleeping

However, sometimes flu can look like other types of illness, especially in children, so it can be hard to recognise:

  • diarrhoea
  • pain in the abdomen (stomach)
  • loss of appetite
  • nausea and vomiting

Compared to a common cold, the fever develops more suddenly, and the symptoms are generally more severe and last longer.

It is the complications of flu that are dangerous. The most common complication is a bacterial chest infection, which can develop into pneumonia. Other complications include:

  • Middle ear infection (otitis media)
  • Septic shock (a severe and life-threatening infection of the whole body)
  • Meningitis (inflammation of the covering of the brain). See the Meningitis Research Foundation website for more detailed information on the signs and symptoms of meningitis.
  • Encephalitis (inflammation of the brain)

Serious complications can affect anyone, even healthy people. They are more common in babies under six months, older people, those with certain long-term medical conditions, and pregnant women. During pregnancy, the baby may be affected, causing premature birth, low birth weight, or even death.

In the UK an average of 600 people a year die from complications of seasonal flu. However in 2013-14, it is estimated that about 11,000 people died from flu-related causes.

How is it passed on?

Flu is passed on by coughing and sneezing. A person can be infected either by breathing in airborne droplets directly, or by touching surfaces on which they have landed. The virus can live for around 24 hours on things such as computer keyboards, hand-rails, door handles, and so on. Around 1 in 3 people infected by the flu virus will not show any symptoms, but they can still pass it on to others. Those who do show symptoms will be infectious for a day before symptoms develop, and for a total period of about a week. Children may remain infectious for longer (up to two weeks). They are more vulnerable to flu than adults and more likely to pass on the infection.

What protection is available?

An annual inactivated flu vaccine is offered to the following vulnerable groups: people aged 65 or over, pregnant women, people with a serious medical condition including asthma, people living in a residential or nursing home, carers of people at risk of complications of the flu, and healthcare professionals.

The UK is gradually introducing the nasal flu vaccine for all children. At the moment the focus is on rolling out the programme to all primary schools, and continuing to vaccinate children aged 2 to 4 years.

Good personal hygiene and cleaning of surfaces can also help to prevent the spread of flu.

Three flu stories

Gigi ended up in intensive care after going down with flu, and has been left with long-term effects. Grateful thanks to Shot by Shot for permission to use this film.

Two families talk about losing children to flu infection. Grateful thanks to Families Fighting Flu and Shot by Shot for permission to use these films.

Page last updated: 
Thursday, November 2, 2017