As distance from the equator increases, serum levels of 25-hydroxyvitamin (25-(OH)D), the storage form of vitamin D, fall during the winter months. Since fall and winter are peak seasons for colds and flu, it has been speculated that higher circulating levels of 25-(OH)D may increase resistance to infection and conversely, lower levels in the winter would increase vulnerability. The study was done to see if there was a link between circulating blood levels of 25-(OH)D and the incidence of acute viral respiratory tract infections.