According to research appearing in the British journal, Epidemiology and Infection (2006 Dec;134(6):1129-40), there may be a connection between the flu and vitamin D deficiency. The article cites R. Edgar Hope-Simpson, who noticed the seasonality of influenza epidemics, and the possible relation to solar radiation.

The researchers propose that the seasonal connection between winter and flu season may be vitamin D. Flu season is during the winter, and vitamin D deficiency is common in the winter—due to the lack of solar radiation. Solar radiation triggers vitamin D production in the skin. Ultra violet radiation (found in sunlight) reduces the incidence of viral infections. Cod liver oil, a source of vitamin D, reduces the incidence of viral infections. Treating children with vitamin D reduced the incidence of respiratory infections.

Activated vitamin D is a steroid hormone that has an effect on immunity. It reduces inflammatory chemicals (cytokines) and increases the effectiveness of certain white blood cells. It stimulates potent anti microbial peptides found in white blood cells and in the cells lining the respiratory tract.