The common cold is caused by a virus. Cold viruses are airborne and when they enter the nose they cause inflammation. Once viruses have entered the nose they damage the cells and infection sets in. The infected cells break down and release other viruses, which then multiply. The slightest quantity of viruses (from one to 30 particles) is sufficient to trigger an infection.

Usually, the infection process begins about 8-12 hours after the virus has penetrated the nose. This interval is called the incubation period and has no symptoms. Initial symptoms appear 10-12 hours after the virus enters the nose. Usually, symptoms reach maximum intensity two days after the onset of the infection.

Many people believe that the common cold is due to cold weather or to sudden changes in temperature (such as going from warm, closed rooms to cold outdoors). Actually, this is not true.  In reality, the opposite is true.  Cold temperatures are more conducive to allowing the virus to penetrate the nose. In cold weather, small protective hairs in the lining of the nose become blocked, which allow the virus to enter the nasal passages more easily. In addition, the majority of viruses survive better when  humidity levels are low, as they are during the coldest months of winter.

The common cold is contagious. It spreads with sneezing, coughing and hand contact.