Influenza: What you need to know about flu

The term 'flu' is often misused to describe a range of mild respiratory bugs, but true influenza is a potentially serious condition for some people. Fortunately, there...
13 November 2021


A woman suffering with a fever caused by influenza (flu) infection.


The term 'flu' is often misused to describe a range of mild respiratory bugs, but true influenza is a potentially serious condition for some people. Fortunately, there are vaccines and treatments...

Often you hear people say they have the 'flu', and many of us describe any winter bug we get as 'flu. But, in reality, it's more likely we're suffering from a common cold or one of the flu's many viral cousins.


The flu, short for influenza, is caused by a highly infectious virus. It spreads through the air from infected people coughing and sneezing, multiplies in cells in people's airways, and causes a respiratory infection.

The viral particles that cause flu are tiny, measuring about one ten-thousandth of a millimetre in diameter. The virus consists of a core, containing the genetic material, surrounded by a coat studded with proteins that help the virus to lock on to and invade its target cells. The virus effectively hijacks the infected cell, turning it into a flu factory. Each infected cell can produce thousands of new viral particles. With every cough or sneeze, they spray out in their millions and loiter in the air until they are breathed in by another person.

Three types of flu virus have been identified: A B and C. Types A and B produce essentially identical illnesses, whereas influenza C usually only affects children and produces milder symptoms, more like a cold. But compared to a typical cold, the flu usually lasts longer (up to a week) and causes fever, cold sweats and muscle aches throughout the body, whereas colds don't.

A number of other infections can produce flu-like illnesses, including bacterial pneumonia (more common in people with pre-existing chest problems including asthma or emphysema) and strep throat, which is a streptococcal bacterial infection in the throat.

Meningitis can also begin with flu-like symptoms, including headache, neck stiffness, photophobia (dislike of bright lights), nausea, vomiting, drowsiness, fever and a rash that does not fade when you press a glass against it. If you suspect meningitis, you need to seek urgent medical care.

The number of people who get the flu varies from year to year, but it is estimated to cause about 750,000 deaths around the world every year, with most of these among the elderly. The virus circulates seasonally, peaking in the colder months of the year, particularly in winter and spring, in any given geography.


Flu has a short incubation period; the gap between exposure to the virus and development of symptoms is usually two to three days and rarely more than 5 days. You become infectious about a day before you begin to feel unwell, and the symptoms last three to seven days. In general, if you are symptomatic, you are infectious and should take steps to avoid spreading the infection to others.

Although infection with flu is confined to the respiratory tract (nose, throat and airways) it produces generalised symptoms throughout the body as the immune system tackles the virus. Most people complain of chills and a high fever, muscle aches and pains, feeling very tired, headaches, sore throat and a cough.

In children, flu can also trigger abdominal pain, nausea, vomiting and diarrhoea.

Bouts of gastroenteritis in adults are often misleadingly referred to as 'gastric flu', but they are actually caused by different classes of viruses and bacteria.

In the aftermath of a bout of flu you can feel tired, and occasionally depressed, for several three weeks. This is normal and usually resolves spontaneously.


In most cases, a flu diagnosis can be made on the basis of symptoms alone, especially at times of high prevalence. Nevertheless, sometimes it is useful to confirm this with laboratory tests, for instance in patients who are very unwell, or to rule out whether antibiotics are required (since viral infections do not respond to antibiotics).

Where this is necessary the diagnosis is made by taking a nose and throat swab. A PCR (polymerase chain reaction) test is used to probe the sample for the presence of the viral genetic code, confirming the infection.


The majority of flu cases resolve on their own and without any significant consequences. But some groups are at much greater risk of complications. These include:

Those over 65. This group accounts for the majority of flu-related deaths. With increasing age, the body becomes less robust and the immune system becomes less effective at responding quickly to new infections.

The very young. Infants and toddlers are more likely to be hospitalised when they get the flu, and to develop more severe symptoms than adults. This is because their immune systems have not yet learned to recognise, and neutralise, the virus.

People with chronic diseases. These include heart disease, kidney disease, diabetes, asthma and other respiratory diseases, which lower the body's defences.

Pregnant women. Research has shown that pregnant women and their unborn babies are more likely to suffer from complications if the mother contracts the flu. Together with the increased demands placed upon the body by pregnancy itself, this effect also means that the illness can last up to three times longer than normal. Vaccination against influenza is recommended for all pregnant women as early as possible in their pregnancy.

If you are in one of these risk groups – or care for someone who is – then it's recommended you get the flu vaccine every year.

You should also be vaccinated every year if you are immunocompromised (eg if you are taking immune-suppressing drugs for diseases like rheumatoid arthritis or following an organ transplant) or have HIV.

Anyone can have the flu vaccination – except children under six months of age – and some groups in the community are eligible for a free annual influenza vaccine. They include:

People over 65

Pregnant women

People with medical conditions that can lead to severe influenza, such as heart disease, liver disease, kidney failure, immunocompromise, cancer therapies, solid organ and bone marrow transplants.


Most cases of the flu are uncomplicated and resolve spontaneously. But some there are some complications of infection, and exacerbations of existing conditions, to watch out for. These include:

Asthma. This can become (temporarily) worse. If breathing remains difficult despite increasing inhaler use, see your doctor.

Croup, or laryngo-tracheo-bronchitis. This condition, characterised by a 'barking' cough, occurs in children and can be triggered by flu infection.

Febrile convulsions. In any illness associated with a fever, children between the ages of six months and five years can occasionally experience brief fits, or seizures. This is not the same as epilepsy and resolves spontaneously once the illness passes.

Secondary bacterial infections in the lungs (pneumonia), the middle ear (otitis media, common in children) and sinuses (sinusitis). This is because damage to the airways by the flu virus can enable bacteria to gain a toehold. The history is usually one of a preceding flu infection which begins to get better, followed by a relapse and symptoms in the affected area, including earache, sinus pain or cough and sputum production. Bacterial 'superinfections' like this usually require treatment with antibiotics.

Viral pneumonia. This occurs when the virus itself causes damage to the lung tissue, leading to worsening shortness of breath, a dry cough and, in severe cases, confusion and a bluish discolouration of the skin due to low blood oxygen levels. Viral pneumonia is less common than its bacterial counterpart but is extremely serious. The patient usually needs to be put on a ventilator, and the condition is frequently fatal.

Reye's Syndrome. This rare but potentially fatal condition occurs in children up to age 15 and is triggered by infections with influenza B viruses or the virus that causes chickenpox. It leads to swelling of the brain and abnormal accumulation of fat in the liver. It also produces drowsiness, inactivity or irrational behaviour. Aspirin-containing remedies should not be given to children under 15 because it can increase the risk of Reye's syndrome.

Inflammation of the muscles (myositis), occasionally including the heart (myocarditis). This can sometimes follow infection with flu, particularly in children. Muscles become tender, most noticeably in the legs, and if the heart is involved there may be palpitations, shortness of breath, chest discomfort and rapid pulse. These conditions usually resolve spontaneously, but can be aggravated by exercise. It is therefore advisable to rest completely until the flu symptoms resolve.

Miscarriage. Rarely, a severe bout of flu can trigger miscarriage.

Neurological problems. Very rarely, infection with flu can lead to problems with the nervous system, including viral and post viral encephalitis (inflammation in the brain), and a condition which causes muscle weakness known as Guillain-Barre Syndrome.

Kidney failure. Dehydration caused by insufficient fluid intake can be sufficient to cause kidney (renal) failure in some people, particularly if they have pre-existing kidney disease. However, this is usually reversible with re-hydration.


In uncomplicated cases of flu, the best treatment is rest, lots of fluids and simple over-the-counter painkillers like paracetamol. You need to see a doctor if you have:

A persistent high fever (39 degrees celsius or above)

Rapid breathing or shortness of breath

Dehydration, drowsiness and confusion

Chest pain

Fainting or seizures (in children – see febrile convulsions above).

In general, antibiotics are not effective against viral infections, including the flu, but may be useful if a secondary bacterial infection develops, such as a pneumonia or tonsillitis.

You can also lower your risk of catching flu by:

Not smoking

Staying away from people who have flu

Washing your hands regularly

Maintaining a healthy lifestyle by eating plenty of fruit and vegetables, getting enough sleep, exercising regularly and avoiding excessive alcohol consumption.


The mainstay of flu prevention is the flu vaccine, which contains a cocktail of three or four killed flu viruses to provide protection against the circulating strains of flu A and flu B infections. Protective antibodies against influenza appear in the blood about two to three weeks after vaccination.

Influenza vaccines are about 60-70 per cent effective in an average year.

By analysing samples of flu collected from patients, the World Health Organisation (WHO) continuously monitors the strains of flu that are circulating in the community. This information helps doctors and scientists to decide which viruses should be included in the vaccine for the forthcoming year.

The vaccine is produced by growing the selected strains of the virus either in cultured laboratory cells, or in hen's eggs. The virus is harvested and chemically inactivated to prevent it from causing disease. This means that individuals with an allergy to eggs should seek medical advice before immunisation.

Because the vaccine kick-starts the immune response in the same way that the virus would, it can sometimes produce mild flu-like symptoms for a day or so following administration. But because the vaccine does not contain live flu virus, it cannot cause you to develop flu.

In some countries, live attenuated flu vaccines have been developed for use in children. These are administered as drops delivered into the nose. The weakened strains of flu in these vaccines produce a very mild infection that is usually asymptomatic but nonetheless powerfully activates the immune system, leading to the formation of protective antibodies and white blood cells.

Live attentuated flu vaccines are being used in children aged up to 16 and pose no risk in health individuals although, in common with all live vaccines, they should not be administered to immunocompromosed individuals, pregnant women or when an individual is suffering from another infection.


The symptoms of influenza can be somewhat alleviated with the use of anitivral drugs that were specifically designed to combat influenza. One is taken orally by capsule (Tamiflu) and the other is inhaled (Relenza). Both antivirals are most effective when taken within 48 hours of symptoms occurring. These drugs are also sometimes used prophylactically to reduce the risk of outbreaks in settings like hospitals and care homes.


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