The term 'common cold' is used to describe an upper respiratory tract infection of viral origin. Many different viruses can cause a cold, sometimes several at a time, which is the main reason it has not been possible for scientists to find a cure.

Symptoms usually come on suddenly and include sneezing, a runny or congested nose, a sore throat, coughing and a general feeling of being unwell, which often worsens for a couple of days before beginning to improve. Colds often last longer than a week and many people find that they have a lingering cough for quite a while after all of the other symptoms have disappeared.

Influenza, often referred to as flu, is also a viral infection and can cause similar symptoms to the common cold. However, people with flu are more likely to suffer from headaches, tiredness, aching muscles and fever, while nasal symptoms such as sneezing and a runny nose are much less prominent than they are during a cold. People with flu are also more likely to loose their appetite and suffer insomnia.

They may feel that their eyes become more sensitive to light and feel hot or sore. The Government tracks the number of people with flu each year, and if it exceeds a certain threshold, it is considered an epidemic. Colds and flu are both spread by droplets expelled during coughing or sneezing that are then inhaled by another person, or passed on via direct contact or sharing objects.

Sufferers, particularly children, may remain contagious for several weeks after their symptoms have gone. While most cases of colds and flu clear up on their own and cause no further problems, complications can affect a minority of people.

After a cold, inflamed and infected sinuses (sinusitis) are the most common problem in adults and older children, although some people may develop pneumonia or acute bronchitis, particularly if they are predisposed because they smoke, are elderly, have poor immunity or have a preexisting respiratory condition such as asthma.

For younger children, acute middle ear infection (otitis media) is the most common complication, with croup, bronchiolitis and pneumonia tending to affect infants. Asthma flare-ups may also occur after a cold or flu. Flu sufferers who experience a complication may develop acute bronchitis, pneumonia, sinusitis or otitis media, while pregnant women may go into premature labour, have a baby with a lower than expected birth weight, or may even suffer a stillbirth or miscarriage.

A cough is a symptom rather than a condition in its own right, and is the result of a reflex response to the airways being irritated. Coughs can be classified as acute, if they are present for less than three weeks, sub-acute, if they last between three and eight weeks, or chronic, if they continue for longer than eight weeks. Dry coughs €“ felt as a tickly throat €“ are often caused by an infection in the upper respiratory tract such as a cold, whereas a chesty cough €“ sometimes described as productive because of the phlegm that is produced €“ can indicate a problem further into the chest, such as a chest infection, mucus draining from the nose or lung damage caused by smoking.