The common cold is a self-limited contagious illness that can be caused by a number of different types of viruses. The common cold is medically referred to as a viral upper respiratory tract infection. Symptoms of the common cold may include cough, sore throat, nasal congestion, runny nose, and sneezing. More than 200 different types of viruses are known to cause the common cold, with rhinovirus causing approximately 30%-35% of all adult colds.

Other commonly implicated viruses include coronavirus, adenovirus, respiratory syncytial virus, and parainfluenza virus. Because so many different viruses can cause a cold and because new cold viruses constantly develop, the body never builds up resistance against all of them. For this reason, colds are a frequent and recurring problem. In fact, children in preschool and elementary school can have six to 12 colds per year while adolescents and adults typically have two to four colds per year. The common cold occurs most frequently during the fall and winter months.