The common cold and influenza (flu) are viral infections of the upper respiratory tract (nose, throat, ears and sinuses).

Although flu tends to be more severe, with symptoms throughout the body, many people get just a mild dose that may seem like a cold. Sometimes, although not often, a severe cold can make a person feel particularly unwell and cause more extensive symptoms than just in the nose and throat, especially if a secondary bacterial infection develops, such as otitis media (an infection of the middle ear) or sinusitis.

However, there are some differences that can give you a clue as to whether it's a cold or the flu.

Symptoms

Colds often start with a scratchy sore throat, a tickly or blocked nose, and sneezing. Symptoms tend to build up slowly and remain limited to the head and face.

Flu, on the other hand, often begins with a more generalised feeling of being unwell. Fever and chills set in, often accompanied by a headache and aching muscles.

Other possible symptoms include extreme tiredness, nausea and vomiting. However, there may be few or no symptoms in the nasal passages or throat.

A person with flu tends to have quite a high fever (usually over 38°C and often higher than 39°C). If you've got a cold, your temperature tends to be normal or only slightly raised, although children may become feverish.

Timing

When a person develops the illness may also be a guide to its cause.

The peak season for colds is autumn and winter, when people spend more time indoors in close contact with others. Central heating also dries out the mucous membranes of the nose, making them more susceptible to infection.

However, colds can develop at any time in the year - you may catch one in the middle of summer - and because colds can be caused by any one of hundreds of viruses, it's quite common to get several in one year.

Flu tends to follow a much more limited season, typically between mid-October and mid-March. If you get symptoms outside this time, it's much less likely to be flu.

There are three major types of flu virus: A (often the cause of flu epidemics), B and C. The virus is constantly changing its structure, so new strains appear each year. We don't have immunity to new strains, which is why we can catch flu repeatedly year after year. However, it's rare to get flu more than once a year.

Other indicators

Most people keep on eating when they have a cold and don't notice much difference in their appetite, although they may prefer foods that don't scratch their throat.

With flu, it's quite common to lose interest in food for several days and even feel quite nauseous or vomit (people sometimes call this gastric flu, but it can be a feature of any flu case).

Cold symptoms usually start to improve after two to three days and are gone within a week. Flu can drag on for longer, and it's often a week or two before a person feels well again. For some, the exhaustion of flu lasts several weeks.

The complications of flu also tend to be more severe. Problems such as pneumonia are a particular risk for people whose immune systems are suppressed and those with chronic lung or heart disease.

Treatments

Most colds need no specific treatment other than painkillers and simple measures such as decongestant rubs or vaporisers. Antibiotics are of no benefit.

Although most people with flu don't need specific treatment, anti-flu drugs such as oseltamivir can shorten the course of the illness or may help to reduce the risk of catching it in the first place. These drugs can be particularly useful for those who are at high risk from the complications of flu.

A flu vaccine is also available. This isn't 100 per cent effective at preventing the illness as the virus changes constantly and the vaccine is developed from the previous year's strains. There is no vaccine against colds.

This article was last medically reviewed by Dr Trisha Macnair in November 2007