Influenza does not follow the predicted patterns for infectious
diseases. In fact, there are several conundrums associated with
influenza epidemics, such as:

1. Why is influenza both seasonal and ubiquitous — and where is the virus between epidemics?

2. Why are influenza epidemics so explosive?

            3. Why do epidemics end so abruptly?

4. What explains the frequent coincidental timing of epidemics in countries of similar latitudes?

5. Why did epidemics in previous ages spread so rapidly, despite the lack of modern transport?

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.