What is the Real Cause of Influenza Epidemics?

A theory gaining weight in the scientific community explains influenza epidemics as a result of a dormant disease, which become active in response to vitamin D deficiency. This theory provides answers for many of the above questions. A disease that remains dormant until vitamin D-producing sunlight exposure is reduced by a winter or rainy season would explain a widespread seasonal disease with a rapid onset and decline.
There is compelling epidemiological evidence that indicates vitamin D deficiency is just such a "seasonal stimulus." Recent evidence confirms that lower respiratory tract infections are more frequent, sometimes dramatically so, in those with low levels of vitamin D. Researchers have also found that 2,000 IU of vitamin D per day abolished the seasonality of influenza, and dramatically reduced its self-reported incidence.

http://articles.mercola.com/sites/articles/archive/2008/11/13/what-is-th...