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Risk Factors and Causes


Dr. Flemings
Upper respiratory tract infections, such as the common cold, are most commonly caused by viruses, but they can also be caused by bacterial infection. Captain Miller, can you tell us more about how these viruses and bacteria spread and cause infection?

Captain Miller
Of course, Dr. Flemings. The viruses and bacteria that cause upper respiratory infections, or URIs, are often highly contagious. They can spread through tiny air droplets that are released when an infected person sneezes or coughs. They can also spread through direct contact with a surface that has been exposed to the virus or bacteria, such as a doorknob, hand railing, or toy. Most people with the common cold are contagious for the first two to three days of their illness.

Upper respiratory tract infections can happen at any time, but they most frequently occur in the fall and winter months. Crowded conditions are more common during these months, as school and cold weather keep people indoors and in groups more often. In addition, many viruses thrive in the lower humidity of winter.

Normally, nasal hairs and wet mucus trap foreign particles and microbes to protect the upper respiratory tract. In addition, the adenoids and tonsils, which are located in the pharynx, or throat, release white blood cells and antibodies that fight and destroy invading microbes.

However, many viruses and bacteria are able to adapt in order to resist destruction. For example, some microbes produce toxins that impair the body's natural defenses. Others change their shape or outer structural proteins to disguise themselves from being recognized by the immune system.

Infection occurs when an invading virus or bacterium overcomes the body's natural defenses and invades the mucous membrane, or inner lining, of the upper respiratory tract.