Many viruses can cause a flu-like illness. There is usually a seasonal outbreak of flu (influenza) in the UK each winter. Flu-like illnesses typically cause a high temperature, aches and pains in muscles and joints, a cough and various other symptoms.
What are flu and flu-like illnesses?
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.
There are three types of influenza virus - A, B and C. Influenza A and B cause most of the cases of flu. Each winter a different type of influenza virus causes an outbreak of flu which affects many people. This is called seasonal flu. If you get a flu-like illness during an outbreak of seasonal flu, it is likely to be caused by the prevailing influenza virus. Most cases of flu usually occur in a period of six to eight weeks during the winter.
Swine flu is caused by a particular strain of influenza A virus which is called H1N1v. It seems to affect children and young adults more commonly than those over the age of 60 years. Most people with this type of flu have a mild flu-like illness. You are more likely to have sickness and/or diarrhoea with this type of flu.
Note: bird flu (avian influenza) is different and is more serious.
Common flu symptoms in adults and older children include:
- High temperature (fever).
- Aches and pains in muscles and joints.
- A dry cough.
- Sore throat.
- Feeling sick (nausea).
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.
Common flu symptoms in babies and young children include fever, sweats, a cough, sore throat, sneezing, difficulty in breathing, lack of energy (lethargy) and 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.
How do you catch flu?
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.Feature
Even if you have never had a day’s illness in your life, your chance of catching flu increases as you get older.— Dr Laurence Knott, Why you should consider the flu vaccine
Serious illnesses that are similar to flu
Other serious illnesses can have similar symptoms to flu (influenza) when they first develop - for example, meningitis, malaria, or pneumonia. If you have a more serious illness, other symptoms usually develop in addition to those mentioned above.
Symptoms to look out for which may mean that you have a different and more severe illness include:
- Rash - in particular if dark red spots develop that do not fade when pressed.
- Stiff neck - particularly if you cannot bend your neck forward.
- A headache that becomes worse and worse.
- Dislike of bright lights - if you need to shut your eyes and turn away from the light.
- Drowsiness and/or confusion.
- Repeatedly being sick (vomiting).
- Chest pains.
- Coughing up blood or blood-stained phlegm (sputum).
Note: it is important to tell a doctor if you have flu-like symptoms and you have been to a country within the previous year where malaria is present. Initial symptoms of malaria can be similar to flu.
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. There are several treatment options as outlined below.
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 (see list below). 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 caused by a germ (bacterium) or pneumonia (see below).
Admission to hospital
A small number of people with flu become ill enough to need hospital admission. This is usually because they have developed complications from flu.
Complications of flu
If you are normally well then you are unlikely to develop complications. You are likely to recover fully. However, see a doctor if symptoms change or become worse. Complications are more likely to develop if you are in any of the at-risk groups listed below.
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 1-2 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).
Further reading and references
PHE guidance on use of antiviral agents for the treatment and prophylaxis of seasonal influenza; Public Health England (October 2018)
Influenza: the Green Book, Chapter 19; Public Health England (August 2015)
HPA guidance on use of antiviral agents for the treatment and prophylaxis of influenza 2011-12; Health Protection Agency (December 2011 - Reviewed October 2012) (archived content)
Jones JT et al; Neuraminidase inhibitors for preventing and treating influenza in healthy adults and children (Review), The Cochrane Collaboration, 2013