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Colds are minor infections of the upper respiratory tract, including the nose and throat. They are common illnesses in the United States, with some authorities reporting an average of 1 billion cases of the common cold every year.

Many different types of viruses cause colds. Typically, viruses are relatively harmless and live in the body for about one week, although they can sometimes linger. Colds are caused by viruses different from those that cause influenza, and colds tend to be less severe than the flu. Children and people in poor health are most vulnerable to colds.

In the United States, most colds occur between September and May, when people tend to spend more time indoors, around others. This time of the year may also be more conducive to colds because of lower humidity (which helps colds to thrive) and because cold weather dries out the mucous membranes lining the inside of the nose, making the body more vulnerable to infection.

Cold viruses are highly contagious. People who have colds are most contagious during the first two to four days after symptoms appear and may be contagious for up to three weeks. The virus particles associated with colds can travel up to 12 feet (3.7 meters) through the air when an infected person coughs or sneezes.

Colds are often transmitted when a healthy person touches an infected person or an object (e.g., toy, phone, towel, utensils) that contains droplets of the cold virus. Once the cold virus is on the person’s fingers, it can then be introduced to the body through touching the eyes, nose or mouth. Infection also can be transmitted by inhaling infected airborne droplets.

The average adult gets two to four colds per year, according to the National Institutes of Health (NIH). Estimates vary for children, but according to the NIH, children experience between six and 10 colds each year, and school-age children may experience even more. The higher incidence of colds among children is due to a number of factors. Children have a less mature immune system and have not yet developed resistance to many viruses. They also spend long periods of time in close proximity to other children who are potentially infected. Finally, children tend to be less careful about washing their hands and adhering to other precautions that can help prevent cold transmission.

As people age, they tend to experience fewer colds as a result of their increased resistance to various viruses. People over the age of 60 average less than one cold per year, according to the NIH. However, certain people may remain at high risk for colds, including people who spend a lot of time in close contact with others, have allergies that affect the nasal passages, or are under psychological stress.