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Colds are minor infections of the upper respiratory tract that can be caused by hundreds of different viruses. They are among the most common illnesses in the United States, with some authorities reporting an average of 1 billion cases of the common cold every year.

Cold viruses are highly contagious and are often transmitted when a healthy person comes into close contact with an infected person or touches an object (e.g., toy, phone, towel, eating utensil) that contains droplets of fluid contaminated with the cold virus. Once the cold virus is on the person’s fingers, it can be introduced to the body through touching the eyes, nose or mouth. Infection also can be transmitted by inhaling infected droplets.

The average adult gets two to four colds per year, according to the National Institutes of Health (NIH). Children, including newborns, experience between six and 10 colds each year. As people age, they tend to experience fewer colds as a result of their increased resistance to various viruses.

The rhinovirus is the most common source of colds, causing between 30 and 35 percent of all adult cold illness, according to the NIH. Coronaviruses are also known to trigger many adult colds, especially during the winter and early spring. More severe viral sources of colds include the parainfluenza virus and respiratory syncytial virus.

Cold symptoms tend to appear about one to three days after the virus enters a person’s body and may include congestion, a stuffy or runny nose, and sneezing. These symptoms typically last for about a week before clearing, although some colds may persist for longer periods of time.

Certain over-the-counter medications may help reduce the symptoms associated with colds. However, the drugs do not shorten the duration of colds and many experts advise against using them because of side effects they may cause.