Flu (Influenza)
Flu (influenza) is caused by the influenza virus. However, many other viruses can cause an illness similar to flu. It is often difficult to say exactly which virus is causing the illness, so doctors often diagnose a flu-like illness.
The illness caused by the influenza virus tends to be worse than illnesses caused by other viruses which cause a flu-like illness. Even if you are young and fit, flu can make you ill enough to need to go to bed.
Flu occurs most often in winter in the UK and peaks between January and March.
The disease is characterised by the sudden onset of fever, chills, headache, myalgia and extreme fatigue. Other common symptoms include a dry cough, sore throat and stuffy nose. For otherwise healthy individuals, influenza is an unpleasant but usually self-limiting disease with recovery usually within 2 to 7 days.
The risk of serious illness from influenza is higher amongst children under 6 months of age, older people, those with underlying health conditions such as respiratory or cardiac disease, chronic neurological conditions, and immunosuppression and also in pregnant women
Symptoms
Common flu symptoms in adults and older children include:
- high temperature (fever)
- sweats
- aches and pains in muscles and joints
- a dry cough
- sore throat
- sneezing
- headache
- feeling sick (nausea)
Common flu symptoms in babies and young children include:
- fever
- sweats
- a cough
- sore throat
- sneezing
- difficulty in breathing
- lack of energy (lethargy)
- poor feeding
Some young children with flu may have a febrile convulsion. A febrile convulsion is a fit that occurs in some children with a fever.
Typically, symptoms are at their worst after 1-2 days. Then they usually gradually ease over several days. An irritating cough may persist for a week or so after other symptoms have gone. Most people recover completely within 2-7 days
Cause
There are 3 types of influenza virus: A, B and C. Influenza A and influenza B are responsible for most clinical illness. Influenza is highly infectious with a usual incubation period of 1 to 3 days.
Influenza A viruses cause outbreaks most years and it is these viruses that are the usual cause of epidemics. Influenza B tends to cause less severe disease and smaller outbreaks overall, and often affects children.
Flu is passed from person to person by droplets created when someone with the infection sneezes or coughs. You can also catch it by touching a surface where the virus has been deposited. Flu can spread quickly in these ways.
To reduce the risk of spreading flu:
- wash your hands often with warm water and soap
- cover your mouth and nose with a tissue when you cough or sneeze
- bin used tissues as quickly as possible
Diagnosis
It is not usually necessary to have an accurate diagnosis of flu as the symptoms can be very similar to other viral infections and treatment advice will be the same.
Your GP will not be able to tell you definitely whether you have flu or another virus infection. However, you are more likely to be suffering influenza during the winter months when we know influenza virus is circulating.
Testing for influenza virus can be done in hospital, by testing a swab from your nose or your sputum
Treatment
Your immune system will usually clear viruses that cause flu (influenza) and flu-like illnesses. Treatment aims to ease symptoms until the infection goes, and to prevent complications.
Stay at home as much as possible to prevent passing on the infection.
Paracetamol and/or ibuprofen can lower your temperature and also ease aches and pains. Drink plenty of fluids to prevent lack of fluid in the body (dehydration). It is best not to smoke. Decongestant drops, throat lozenges and saline nasal drops may be helpful to ease nose and throat symptoms.
Note: parents and carers should not use over-the-counter cough and cold medicines in children under 6 years old. There is no evidence that they work, and they can cause side-effects such as allergic reactions, effects on sleep, or even hallucinations.
Antiviral medicines called oseltamivir (trade name Tamiflu) and zanamivir (trade name Relenza) are sometimes used. Antiviral medicines do not kill the virus but interfere with the way the virus multiplies.
Antiviral medicines do not cure flu or offer long-term protection against flu. If you do not have an antiviral medicine, you are still likely to make a full recovery. However, antiviral medicines may reduce the risk of developing complications. They may also reduce the severity and duration of symptoms by a day or two.
An antiviral medicine may be prescribed if you are at increased risk of developing complications when you have flu. Treatment is usually taken for five days.
GPs are only allowed to prescribe an antiviral medicine when national surveillance schemes show there is a lot of flu in the community. An antiviral medicine is also often used in people who are admitted to hospital with flu.
Medication may also be prescribed to certain people to prevent flu – for example, if you live in a residential home and there is an outbreak of flu in the home. It is also given if you are at increased risk of complications and have been in close contact with a person with flu.
Antiviral medicines should be given within 48 hours of getting the flu (within 36 hours for zanamivir in children) or of having contact with someone who has the flu. This is because the sooner they are given, the better they work.
Antibiotics kill germs called bacteria, but not viruses. Therefore, they are not routinely prescribed for viral illnesses such as flu or flu-like illnesses. However, they may be used if a complication develops like a chest infection or pneumonia
Prevention
Flu vaccination is offered free if you:
- are aged 50 or over
- have certain health conditions
- are pregnant
- are in long-stay residential care
- receive a carer’s allowance, or are the main carer for an older or disabled
- person who may be at risk if you get sick
- live with someone who is more likely to get a severe infection due to a weakened immune system, such as someone living with HIV, someone who has had a transplant, or is having certain treatments for cancer, lupus, or rheumatoid arthritis
Healthy children are offered immunisation through a nasal spray, rather than an injection. Immunising your child will protect them but also cut the risk of them passing on the infection to other vulnerable members of the family.
Vaccination remains the best protection we have against the flu virus.
If 10 people have the flu jab, it usually provides protection from flu for 7 or 8 of them.
The immunisation is best given in the autumn before flu starts spreading, but you can get vaccinated later. It takes about two weeks to make a response to the flu vaccine.
The flu injection is made from the three or four strains of flu virus that are most likely to cause outbreaks in the coming winter.
Each year these are slightly different, so a new jab needs to be made every year. You need a yearly jab to stay protected.
When to contact your GP
If you are normally well then you are unlikely to develop complications. You are likely to recover fully.
However, you should ask for an urgent GP appointment or get help from NHS 111 if you or your child have symptoms of flu and:
- you are worried about your baby’s or child’s symptoms
- you are 65 or over
- you are pregnant
- you have a long-term medical condition – for example, diabetes or a condition that affects your heart, lungs, kidneys, brain, or nerves
- you have a weakened immune system – for example, because of chemotherapy or HIV
- you have a rash
- your headache is getting consistently worse
- you are having difficulty breathing or chest pain
- your symptoms do not improve after 7 days
The most common complication is a chest infection caused by a germ (bacterium). This may develop in addition to the viral infection (that is, a secondary infection). This can sometimes become serious and develop into pneumonia. A course of antibiotic medication will often cure this. However, a bacterial infection can sometimes become life-threatening, particularly in those who are frail or elderly.
Note: with flu (influenza) or a flu-like illness it is common to have a cough that lingers for 2-3 weeks after other symptoms have gone. Green phlegm (sputum) does not necessarily mean that you have a secondary chest infection.
The symptoms to look out for that may indicate a secondary chest infection include:
- a recurrence of a high temperature (fever)
- worsening of cough
- shortness of breath
- fast breathing
- chest pain
Other complications that sometimes occur include a sinus infection and an ear infection. Other serious complications are rare, such as brain inflammation (encephalitis).