Causes of Colds and Flu
Flu is caused by the influenza virus and each winter a different strain of the influenza virus causes an outbreak of flu. The common cold is caused by over 200 different viruses, mostly belonging to a group known as rhinoviruses.

Viruses replicate in the cells which line the nose, and are then spread from person to person through coughing and sneezing. They can also be spread by contaminated fingers, which pass the virus to the nose and eyes. Fingers can easily become contaminated by a cold or flu virus, for example by touching door handles or telephones that are contaminated with droplets of the infected mucus. Patients will be most infective when they have the early symptoms of a cold - sneezing, runny nose or cough.

Why are colds and flu more common in winter?
Cold and flu can occur all year round, but they are more common during the autumn and winter months. One reason for this is thought to be because people spend more time crowded indoors in poorly ventilated rooms, where viruses are more easily spread. People are also more susceptible to colds and flu if they are tired and run down, or if their immune system is weakened in any way.