Medical information for patients

Influenza ('flu)

Influenza is a virus that causes a severe form of respiratory tract infection with generalised bodily symptoms. It spreads around the world in epidemics and is responsible for much ill health as well as many deaths.


If you start to develop cold symptoms but starting more rapidly and rather more violently, with higher fever and severe aches and pains, often in the back and muscles, then you may well be developing influenza. This may be associated with severe headache, cough, and, as a result of the fever, intermittent sweating and shivering. Sometimes there is a gastrointestinal element, with vomiting or diarrhoea.

Many people think they have had "flu" when all that they have suffered is a bad cold. When you have influenza you will know the difference. Most people will find it impossible to leave their bed and feel terrible.

The worst symptoms usually last for three to five days, and then should begin to improve. It is common to need two to three weeks off work as there is considerable debility left after the feverish illness is over. Do not be surprised to be quite depressed; this is a natural after-effect of the condition.

Influenza makes everyone feel terrible, but most people recover. However it does have a small but significant mortality, especially in the very young, the very old, and those with poor immunity.


Influenza is caused by a virus which attacks our body's cells, resulting in various effects depending on the strain of the virus.

The virus mutates all the time and new variations (strains) arise. Unfortunately immunity against one strain (which is conferred by exposure or immunisation) does not protect against other strains. In the era of rapid air transport the worldwide spread of a new type of influenza can be extremely fast.


Diagnosis is largely based on the severity of the condition and the way in which it affects you. Influenza is more likely if there is an epidemic at the time. It is possible to test body secretions for the influenza virus, but this is not usually of practical value as the test takes some time to show a result.


Until recently there was no specific treatment for influenza, and for most people it is still best to treat the symptoms in the old fashioned way:

New Developments

There are very few treatments that work on viruses. Those that do work on influenza need to be given within 48 hours of the onset of the attack, before it is possible to be absolutely sure of the diagnosis, and only shorten and reduce the severity of the condition.

The other aspect of this is that they are only available on prescription, and if everybody with influenza were to see their doctor within two days of developing symptoms the healthcare systems in most countries, already stretched by an epidemic, would probably grind to a complete halt. Prevention, with influenza vaccine, is a far better option.

There are now three influenza treatments licensed for use in the UK:

Oseltamivir and zanamivir belong to a new class of drug (neuraminidase inhibitors) and are licensed for treatment of both the main types of influenza in humans (type A and type B). Amantadine has the benefit of being less expensive, but only works on type A.

The way in which these may be used within the National Health Service in England & Wales is set out by the National Institute for Clinical Excellence (NICE). Under the NICE guidelines:

When to call the doctor

You should consult your doctor if symptoms such as clouded/altered consciousness, breathlessness, or severe chest pain occur, and if you are concerned. Try not to call just because of the temperature, aches and pains, and cough, as there is nothing your doctor can do that you can not, and what is more, there are likely to be many other people with similar symptoms, stretching family doctor, nursing and hospital services to the limit.

If symptoms continue unabated for more than a week, or if you seem to be improving for a few days then get much worse again, it is worth asking your doctor's advice.

Sometimes, during 'flu, a secondary infection gets in while the body's defences are down. This is most commonly a chest infection and can be a pneumonia. The secondary infection is often caused by bacteria, and as a result is likely to respond to an antibiotic.


There are many influenza viruses, often named after places where epidemics have occurred. There tend to be only one or two that reach epidemic proportions each year. By monitoring the progress of these worldwide, the World Health Organization (WHO) each year decide on the best "cocktail" of influenza types to be included in that year's influenza vaccine.

A new vaccine is being developed that is said to be able to allow for different and new strains. This will have the benefit of lasting much longer when it is introduced. It may then become cost effective for the NHS and other healthcare systems to consider widespread immunisation.

It is advised that people belonging to the following risk categories consider annual influenza vaccination. In the UK this is available from your doctor from October.

High risk groups

There are various groups who are more at risk during influenza epidemics. These include:

Other people who are not at such risk from influenza might choose to have the vaccination, but in the UK this is not routinely covered the National Health Service.

The NICE guidelines recommend the use of oseltamivir to prevent influenza in the limited number of cases where:

Further information

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