Influenza, simply called ‘flu’, is a serious viral infection that affects the upper airways and lungs. Flu is highly infectious with a usual incubation period of one to three days. Usually, most healthy people recover in a week or so, though flu often leaves a person feeling tired and ‘out of sorts’ for a while afterwards. Flu can, however, lead to serious medical complications especially in the elderly and people with underlying medical conditions.

Flu or cold?

Many of the symptoms of flu are similar to those of the common cold, and many people incorrectly refer to a heavy cold as flu. Symptoms of a cold appear gradually, are not severe and affect just your nose and throat.

Flu is a far more serious than a cold, and can cause complications for people who have particular risks. A flu vaccine is available free of charge from the NHS to everyone 65 or older, to women who are pregnant, those who have certain medical conditions, or work within the groups or communities listed as ‘at risk’ within the annual Department of Health Influenza Immunisation Programme.

There are vaccines available that can protect against flu, you can check whether you should have the flu vaccination, by clicking here.

The information outlined above reflect current NHS Choices recommendations

Symptoms

Symptoms of flu are more severe than colds, causing fever and aching muscles. You will not be able to do your usual activities.

Seasonal flu can give you any of these symptoms:

• sudden fever (a temperature of 38°C/100.4°F or above)
• dry, chesty cough
• headache
• tiredness
• chills
• aching muscles
• limb or joint pain
• diarrhoea or stomach upset
• sore throat
• runny or blocked nose
• sneezing
• loss of appetite
• difficulty sleeping

Babies and small children with flu may also appear drowsy, unresponsive and floppy.

Symptoms will usually peak after two to three days and you should begin to feel much better within five to eight days. A cough and general tiredness may last for two to three weeks. You can also get advice from NHS Direct 0845 4647.

What’s in a name?

Flu can be very dangerous, even for young and healthy people, particularly when a new type develops and nobody has any immunity to it. When this happens the infection can spread very rapidly around the world and many millions of people can be affected and deaths can occur. This is called a pandemic.

The names given to different types (‘strains’) of flu viruses are related to the area and year in which the new type was first found – names like ‘Asian flu 1957’ or ‘Spanish flu 1918’.

A new virus may be able to pass from an animal or bird to humans, but not from one human to another, so the only people who will catch it are those who are in contact with an infected animal or bird. It is only when a new strain of virus changes again so that it can pass from human to human that there can be widespread infection of people – an ‘epidemic’ or ‘pandemic’.

The medical system for identifying different Influenza A viruses is based on changes in two particular parts of the virus called H and N, which are the ‘markers’ (‘antigens’) that the body uses to recognise the virus and develop the right antibodies to protect itself. Examples look like this: A (H1N1), A (H1N2) and A (H3N2).

Epidemics and pandemics

Flu is usually more common in the winter months. In the UK, the ‘flu season’ roughly lasts from September to April. When a large number of people get the flu then an epidemic occurs – in the UK this is when more than 200 out of every 100,000 of the population consult their doctor about flu. If a large proportion of the world population catches a strain of flu to which they have little immunity and which spreads very rapidly it is known as a pandemic, and the flu may cause a high number of deaths even among people who would not normally be seriously affected by flu.

In 1918 the Spanish flu pandemic killed more fit and healthy young men than the First World War itself, and caused the death of up to 40 million people worldwide. Since then, there have been two more flu pandemics, one in 1957/58 and another in 1968/69, although these were not so severe.

Avian (‘bird’) flu

There has been a concern about the spread of avian (‘bird’) flu, and the number of people who have caught it is closely monitored. As of 29 December 2010, there have been a total of 512 cases mainly in Asia, with 304 deaths. This strain of flu, H5N1, does not spread from human to human, but can only be caught from infected birds. If, however, it did combine with a human strain of flu in a susceptible person or an animal, this could result in a new strain of flu that could spread among humans and lead to a pandemic.

Swine flu

More recently, a strain of H1N1 flu called 'swine flu' has caused concern because it is a new variant of flu that has passed from pigs to man and become a human flu virus. This means that it can pass from human-to-human and has led to infection rates in some parts of the world now qualifying as pandemic levels with most people having little immunity to it.

A number of deaths from swine flu were reported in Mexico, where it was first found, however most cases in the UK have proved to have relatively mild symptoms and to be self-limiting.

Although in August 2010, the World Health Organization declared that the swine flu (H1N1) pandemic was over, it did not go away. As predicted by the WHO, the H1N1 virus has taken on the behaviour of a seasonal flu virus and is likely to continue to circulate for some years.

The WHO also anticipated that ‘localised outbreaks’with ‘significant levels of H1N1 transmission’ might occur. In 2011, the UK saw an increase in the numbers of people with H1N1 flu, alongside those having other strains of seasonal flu.

Because of the changing nature of influenza viruses, the World Health Organization monitors changes in flu viruses throughout the world. Each year it makes recommendations about the strains to be included in vaccines for the forthcoming winter. The H1N1 strain is included in the current trivalent seasonal influenza vaccine.