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Flu (influenza)

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Flu (also known as influenza) is a disease of the lungs and upper airways caused by infection with a flu virus. The virus spreads in the lungs and airways.
Brought to you by NHS Choices



There are three flu viruses, known as A, B and C.

The main symptoms are a high temperature that comes on quickly, and general aches and pains. You may also experience a loss of appetite, nausea and a harsh dry cough. Your symptoms will usually peak after two to three days and you should begin to feel much better within five to eight days, although a cough and general tiredness may last for two to three weeks.

The flu virus is usually spread in the small droplets of saliva coughed or sneezed into the atmosphere by an infected person. Direct contact with hands that are contaminated with the virus can also spread infection.

It takes between one and four days (average two days) to go from being infected to having the full symptoms. People with flu are usually infectious a day before symptoms start and remain infectious for approximately five days after the start of the flu symptoms. Children and people with lowered immune systems may remain infectious for longer. You should therefore try to avoid all unnecessary contact with others during the infectious period.

Flu usually occurs during the winter months (from October to April in the UK). Complications such as a chest infection can affect elderly people or people with certain medical conditions. This can result in serious illness and can be life-threatening.

The number of people who consult their GP with flu-like illness during the winter varies considerably from year to year (usually between 50 and 200 for every 100,000 people). However, healthy people with flu do not need to consult their GP.

An epidemic can be declared if more than 400 people per 100,000 of the population consult their GP with flu or a flu-like illness every week. Flu-like illness describes a variety of other illnesses, which result in similar symptoms.


Symptoms of flu

Flu is an infectious disease of the upper air passages (known as the upper respiratory tract).

The symptoms are:

  • fever that comes on quickly (38-40C; 100-104F),
  • sweating and feeling feverish,
  • general muscle aches and pains,
  • a feeling of general tiredness,
  • dry, chesty cough,
  • sneezing,
  • running or blocked nose, and
  • difficulty sleeping.

Babies and small children with flu can also have the following symptoms:

  • lethargy (drowsy, unresponsive, limp or floppy), and
  • poor feeding.

The symptoms can also include nausea, vomiting, and diarrhoea, especially in children.

Many of the symptoms of flu are similar to those of the common cold, and many people incorrectly refer to a heavy cold as flu. Viruses cause them both, but flu affects the lungs much more that a cold does. Also flu doesn't usually give you the same congested heavy headed feeling that a cold does.


Causes of flu

The flu virus family contains three main types, A, B and C. Flu viruses are known to mutate (change) over time. Also, strains of the flu virus are classified on where and when they were first identified and given a serial number.

Flu A occurs more frequently (every two to three years) and is more serious than type B. It is very likely to mutate and it regularly produces varieties to which populations have no resistance. It is for this reason that widespread epidemics occur that may affect whole continents. These are known as pandemics and are caused by new strains of the type A virus.

Generally, flu B causes a less severe illness, although it is responsible for smaller outbreaks. Flu B is much more stable than the flu A virus and if you have been infected with flu B, your immunity to further flu B infections will last for many years. Flu B mainly affects young children (5-14 years of age) who have not been exposed to the virus and have not developed immunity.

In the winter of 2005/6, the majority of flu activity was confined to type B with only a few cases of flu A reported. Type C usually causes a mild illness similar to the common cold.

In recent years, two subtypes of flu A have been circulating, the H1N1 subtype and the H3N2 subtype - Panama or Moscow-like strains. In 2003/4, the main strain circulating was a flu A (H3N2) Fujian-like strain. This is slightly different to the A (H3N2) Panama-like virus, which has been circulating in the UK in recent years.


Diagnosing flu

You can usually diagnose flu yourself based on the characteristic of your symptoms. It is unusual for any other diagnostic tests to be necessary unless there are complications.

Your GP cannot normally say exactly which virus is causing your flu-like symptoms. The main role of laboratory diagnosis is to identify the main circulating types and strains of the flu virus. To do this, a viral culture is necessary which will be taken from your nose or throat using a swab.

If some other disease is suspected (for example, malaria, if you have recently travelled to an area where malaria is a problem), then other tests or a referral may be necessary.


Treating flu

If you are usually fit and healthy, you can treat the symptoms of flu at home using remedies commonly available from pharmacies. Antibiotics are not prescribed for flu as they have no effect on viruses. However, occasionally it may be necessary to control complications, especially serious chest infections or pneumonia.

You should stay at home and in bed for as long as the acute symptoms persist which is usually for two or three days after the symptoms peak. You should drink plenty of fluids and to treat the symptoms as necessary.

Your GP may prescribe the medicines listed below if you are at risk of complications and Flu type A or B is circulating in the community. Well-developed clinical guidelines exist to guide doctors in the treatment of flu. The National Institute for Clinical Excellence (NICE) have also produced guidelines on the use of specific treatments for people at risk of complications.

Drugs that fight the flu virus are called antiviral medicines. They do not cure the virus but may reduce the severity and length of time that the symptoms last. Zanamivir and oseltamivir are used to treat the flu viruses A and B and work by stopping the virus from multiplying in your body.


Zanamivir is a dry powder that is taken by breathing it in through a device called a diskhaler. You need to start taking it within 48 hours of your first flu symptoms and the dose is two inhalations twice a day for five consecutive days. Most people can safely take zanamivir, although in the UK, it is normally used by people over 65 or those who have medical problems such as asthma, chronic obstructive pulmonary disease (COPD), heart disease, kidney disease, a weak immune system or diabetes. However, it isn't recommended for children under 12 years of age.

Recent studies of zanamivir showed that those who took it improved about one day earlier than expected, including those aged over 65 and those with other medical problems. However, it didn't help prevent complications of flu (such as pneumonia) and it may cause breathing difficulties if you have asthma or lung problems.


Oseltamivir is taken by mouth (orally) in tablet or liquid form. As with zanamivir, you need to start taking oseltamivir within 48 hours of getting the first symptoms of flu. The dose is one tablet twice a day for five consecutive days. However, a smaller dose will be prescribed if you have kidney disease. Most people can safely take oseltamivir, but it is usually taken by people over 65 or those with the medical conditions described above. However, it isn't recommended for children under the age of one.

Oseltamivir can cause nausea, vomiting, stomach pain and diarrhoea. These side effects should not be serious but if they are, you should see your GP. Prevention of flu with any of these medicines is not recommended at the moment except when there is an epidemic. During the flu season, the NHS closely monitors the levels of flu and will notify doctors if other steps need to be taken to prevent the spread of the disease.


Amantadine is another oral medicine, which only works against type A flu virus. It comes in tablet or syrup form and can help prevent flu if you have recently come into contact with the virus. However, at the moment in the UK, amantadine is not recommended to treat or prevent flu (except in exceptional circumstances if the other medicines are not suitable) because there has not been enough research carried out to show whether it helps those who are most at risk from the virus such as the elderly and those with another medical condition. Possible side effects of amantadine are nausea, dizziness and insomnia.


Complications of flu

A secondary bacterial chest infection, which develops in addition to the viral infection, is the most common complication. Occasionally, this can become serious and develop into pneumonia. A course of antibiotics will usually cure this, but the infection sometimes becomes life threatening, particularly in the frail and elderly. Other serious complications are uncommon.

Rarely, flu can develop into a high fever, acute bronchitis, encephalitis and pneumonia. The risks for complications and serious illness are higher among the following groups of people:

  • those aged 65 years and older,
  • babies and infants,
  • people with other medical conditions (for example, long-term heart, lung or kidney disease, diabetes, or those with a weakened immune system due to treatment or disease), and
  • people living in nursing, residential or other long-stay homes where the spread of the virus is more likely.


Preventing flu

Regular immunisation is recommended for people who:

  • are 65 or over,
  • are living in an old peoples home or nursing home,
  • are the main carer for an elderly or disabled person whose welfare may be at risk if the carer falls ill,
  • are over 6 months old and have a chronic condition such as a heart problem, asthma, kidney disease or diabetes,
  • have a lowered immune system due to disease or treatment such as steroid medication or cancer treatment,
  • have chronic liver disease, or
  • work in close contact with poultry - for example, working in areas where poultry are kept for rearing or egg production, handling or catching live poultry, sorting eggs in poultry houses, or slaughtering and cleaning poultry.

The vaccine is available free of charge to these people.

Vaccination should be done in the autumn (October / early November) by injection. Anti-flu vaccines, against recent types of A and B virus, are available and are roughly 75% effective. If you have been vaccinated and get flu, it tends to be a much less serious strain of the disease.

The NHS has targets for the number of people receiving flu injections and the Department of Health measures uptake figures on an annual basis. For those aged 65 or over, the target is 70% and, in recent years, this has been exceeded.

Useful links

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This article was originally published by NHS Choices

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