Humans are the only source of cold viruses, and close contact with an infected person is generally necessary for viruses to be transmitted. A person with severe symptoms early in the course of a cold is much more likely to transmit a cold than is someone whose symptoms are mild or who is in the late stage of a cold. Very high concentrations of virus are found in the nasal secretions and often on the hands of infected people during the first 2 or 3 days of a cold. By the fourth or fifth day, virus levels are often undetectable, but low levels can be present for 2 weeks. A few virions are sufficient to infect the nasal mucosa, but the mouth is quite resistant to infection. In adults the disease is usually contracted when airborne droplets containing virus particles are inhaled. Transmission can also occur when virus-containing secretions are unwittingly rubbed into the eyes or nose by contaminated hands. Virus introduced into the eye is promptly transmitted to the nasal passage via the nasolacrimal duct. With reasonable caution, however, colds are not highly contagious. In a study of non-immune adults exposed in a family or dormitory setting, less than half contracted colds. Young children, however, transmit colds and other respiratory viruses very effectively because they are often careless with their respiratory secretions. Experimental and epidemiological studies show that there is no relationship between exposure to cold temperature and development of colds, contrary to popular belief. Emotional stress, however, can almost double the risk of catching a cold.

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