Flu, or influenza, is a viral illness that occurs predominantly in the winter months. It is easily confused with the common cold, which is caused by a different virus.

Influenza viruses can infect the nose, throat, sinuses, upper airways and lungs. Flu spreads easily from person to person, through droplet distribution when an infected person coughs or sneezes or, quite commonly, by hand-to-hand contact.

Many believe flu is merely a nuisance, much like the common cold, that cannot be prevented. In fact, flu can be a severe and sometimes life-threatening disease. You can avoid it by going for a flu shot every year.


There are three types of influenza viruses: types A, B and C. Influenza A and B generally cause indistinguishable diseases, although influenza B is often milder than influenza A. Influenza C causes a mild respiratory infection in young children. You may be infected with any combination of the viruses in the same season (even, rarely, at the same time).

Humans are occasionally infected by H5 influenza A (bird flu) viruses, but cannot pass these on to other people.


Symptoms start about 48 hours after exposure to the virus, although this period may vary from one to four days. Typically, fever is continuous for about three days, although symptoms such as cough, lethargy and a general feeling of unwellness may persist for longer. Most people have an uneventful recovery after treatment for symptoms only.

A person is considered infectious when the virus is shed from the airways (nose and throat). This can occur from the day of infection and can continue for eight days – longer in people with decreased immunity.

By coughing or sneezing, the virus is transmitted in droplets (most commonly); by infectious aerosols (tiny particles, which can stay in suspension in the air for up to an hour); or on contaminated hands, tissues or other objects. If other people breathe in the droplets or aerosols or touch infected surfaces, the virus can reach the cells of the airways and establish an infection.

Risk factors

Influenza impacts people of all ages. In healthy children, young adults and middle-aged people, the disease is mostly mild. Flu can, however, be life-threatening in older people, babies, toddlers and in people who have certain underlying conditions.

In the general community, preschool and young school children are most likely to get flu. This is because children have little pre-existing immunity and are susceptible to viruses, which they then bring home to their families.

There are high infection rates in old-age homes. Other closed communities such as university campuses and military bases are also prone to outbreaks of flu, which may last a few weeks.

Those at highest risk should do everything they can to avoid the illness, and should be treated as soon as possible if complications develop.


Symptoms can occur abruptly; it is sometimes possible to pinpoint the exact hour they begin.  In uncomplicated influenza, some or all of these symptoms may be present:

    • A high fever (often higher than 39°C) with chills. The fever is highest in children and least marked in the elderly.

    • Dry cough

    • Sore throat

    • Blocked nose or nasal discharge

    • Sweating and shivering

    • Muscle aches and pains, especially in the legs

    • A general feeling of unwellness

    • Fatigue and wanting to sleep all day

    • Children often vomit and have diarrhoea, but this is infrequent in adults.

Both children and adults can develop serious complications when they have flu:

    • Primary influenza virus pneumonia occurs when flu has caused severe lung damage. After the usual flu symptoms, the fever persists, the cough worsens and you’ll become extremely short of breath. In severe cases, you may develop a bluish tinge and feel confused from oxygen lack. This form of pneumonia is very serious, requiring hospitalisation. Most people who develop it have underlying heart or lung disease.

    • Secondary bacterial pneumonia is more common, occurring when bacteria cause a secondary lung infection. Symptoms include recurrence of fever, shortness of breath and productive (secretion-producing) cough four to 14 days after flu symptoms have almost gone. It is usually caused by the bacteria Streptococcus pneumonia, Staphylococcus aureus and Haemophilus influenzae type B. Some doctors recommend that people in high-risk groups be vaccinated against pneumococcal pneumonia as well as flu.

    • Viruses may worsen chronic lung diseases such as chronic obstructive pulmonary disease (COPD).

    • Young children are prone to middle-ear infections (otitis media) whenever they have an upper respiratory tract infection. In most cases, the ear infection is caused by the virus itself and is not a secondary bacterial infection, so antibiotics are seldom necessary.

    • Children may also develop croup (a viral infection of the vocal cords and large main airways) as a result of flu and other respiratory-tract virus infections.

    • In rare cases, mostly in children, inflammation of the muscles of the body (myositis) can occur. Painful, tender leg muscles are a symptom.

    • Also very rarely, the heart muscle may become inflamed (myocarditis). Symptoms include tiredness, shortness of breath, heart palpitations, a rapid pulse and discomfort in the chest. Because myositis and myocarditis are more likely if the muscles are under stress, it’s not a good idea to exercise while you have the flu (or any viral illness).

    • Young children with flu may have fever fits (febrile convulsions). Feverish children should never be given aspirin or medicine containing aspirin. A disease affecting the brain and liver, Reye’s syndrome, can occur in children given aspirin for fever associated with viral infection. Symptoms include vomiting, lethargy, altered consciousness, seizures and respiratory arrest. Most recover, but permanent brain damage or death can sometimes result.

    • Complications of the nervous system and brain. Other (very rare) central nervous system complications associated with flu include Guillain-Barré syndrome, encephalitis and transverse myelitis.


It is hard to accurately determine the prevalence of flu, as many infections are not reported to healthcare institutions; but it is very common. In the United States, for example, an estimated five to 20% of the population are infected yearly: roughly 50 million cases. Worldwide, between three and five million cases of severe illness occur annually, with an estimated 250 000 to 500 000 flu-related deaths. About 21% of people living in the same house as an infected child or adult will contract flu.


Healthcare professionals can usually recognise flu by looking at the symptoms and signs alone.  Other respiratory viruses can cause a similar clinical picture, but if it is known that flu is active in the community, then the flu diagnosis becomes even more probable.

The doctor’s suspicion can be confirmed by a laboratory test. Throat and/or nasal swabs are taken and the virus is grown in cell culture. The virus can then be detected in various ways, eg. by adding red blood cells or by using a fluorescence microscope. Recently developed techniques can detect the presence of the genetic information (RNA) of the virus.

Laboratory diagnosis might be required:

    • When the illness is severe, needing admission to hospital

    • In children, where other viruses mimicking influenza may be the cause of disease

    • When infection with more than one virus is suspected

    • To decide whether an antibiotic is necessary (viral infections don’t respond to antibiotics)

    • For academic interest


If you have flu, you should stay in bed, rest and drink fluids, allowing your body to fight the infection. When the diagnosis is clear and the illness uncomplicated, there is not much else to do. In most cases, symptoms will subside in three to four days, with or without symptom-relieving medication.

In low-risk cases, without signs of secondary bacterial infection, you or your doctor will treat flu mainly with over-the-counter medications. (Caution: Pregnant women should be cautious about taking drugs, and children under 16 should not receive aspirin.) Such medications may help relieve symptoms, but will not fight the virus itself or cure the flu:

    • Paracetamol, aspirin or ibuprofen may help to relieve fever, muscle aches and headache.

    • Decongestants help to treat nasal congestion. Their use for more than five consecutive days will worsen symptoms afterwards, due to a rebound effect.

    • Suppressive cough mixtures may help clear up a dry cough.

    • Vitamin supplements, such as vitamin C and A, and zinc lozenges have been advocated to help treat colds and flu, but there is little evidence for their efficacy against flu in otherwise healthy people. High doses of vitamin C and zinc may cause gastrointestinal side effects such as stomach pain, nausea and vomiting.

    • In vitro tests have shown that spirulina can stop the influenza virus from reproducing.

    • Probiotics can support overall immune function, but this effect has not yet been tested on the influenza virus.

In high-risk cases, antiviral drugs may be prescribed by a doctor:

    • The neuraminidase inhibitors oseltamivir and zanamivir are currently available for treatment and prevention of influenza A and B. Due to the risk of severe bronchospasm, zanamivir should not be used by people with underlying airways disease. It should also be avoided by those with allergic reactions to milk proteins, as lactose powder is used as a carrier.

    • The adamantanes, for the treatment and prevention of influenza A, are no longer recommended as viruses have become resistant to them.

    • Prevention of flu with antivirals should not seen as a substitute for vaccination, and should be reserved for people who cannot be vaccinated or are severely immunosuppressed.

Influenza should not be treated with antibiotics, as it is caused by a virus, not a bacterium. Antibiotics should only be prescribed when secondary bacterial infection is suspected. Remember to complete all courses of antibiotics and antivirals, to prevent the development of resistant infections.

In very severe cases, the doctor might consider hospitalisation of the patient.

Special considerations

    • Influenza and breastfeeding. The flu virus cannot be transmitted from mom to baby through breast milk. Continue breastfeeding if you have flu, as the antibodies you transmit to your baby via the breast milk help to protect him or her from infection.

    • Influenza and pregnancy. Pregnant women, especially those in the second and third trimester, are at increased risk for developing severe seasonal influenza. Current influenza vaccination recommendations include all pregnant women.

    • Influenza and sport. Refrain from strenuous exercise while you’re ill. If you are a professional athlete, remember that several ingredients of over-the-counter medications for cold and flu are banned by the respective governing bodies.

    • Flu and smokers. In smokers, the cilia (the tiny “brooms” of the airways which clear the lungs) are already damaged, which means that an important defence mechanism of the airways is compromised. The flu can make one even more vulnerable to complications such as secondary bacterial infection.

When to see a doctor

    • If you have a high fever for more than a few hours that does not respond to over-the-counter medicine

    • If your fever lasts longer than two days

    • If you feel sick and just don't seem to get better

    • If you have a cough that begins to produce phlegm

    • If you have any sign of complications of flu

    • If you have difficulty breathing or feel a sharp pain when breathing

    • If you fall into any high-risk categories, including people with cancer, people on medication after organ transplantation and HIV-positive people – even if it’s a just mild bout of flu


Although flu is generally not dangerous, it can cause serious complications and even death, especially in the elderly and the very young. You should try to avoid contracting this highly contagious disease, especially if you’re vulnerable.

You can’t really help being exposed to cold and flu viruses. Many are acquired from people who don’t yet show symptoms, and it’s difficult to contain viruses that travel through the air. However, there are general measures you can take to give yourself the best chance of avoiding infection:

Wash your hands frequently and don’t touch your nose, eyes or mouth unnecessarily.

"Contain" sneezes and coughs with disposable tissues, dispose of them right away and wash your hands afterwards.

Try not to touch objects around you when in public places, such as the rail of the escalator or your coughing colleague's pen or computer mouse.

To minimise exposure, avoid close or prolonged contact with people with a cold or flu. With an incubation period of one to four days and a contagious period of seven days or longer, it’s best to avoid any person with flu for at least a week.

There may be a role for vitamin A supplements to prevent flu in malnourished children.

Stop smoking. Smokers are more vulnerable to complications of respiratory infections.

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