Another article illustrating the importance of vitamin D to your health.

"For instance, the researchers point to studies showing that in winter, colds, flu, and other respiratory diseases are more common and more likely to be deadly than they are in summer. During winter, ultraviolet-light exposure tends to be low because people spend more time indoors and the atmosphere filters out more of the sun's rays, especially at mid and high latitudes.

Cannell's group cites a 1997 study showing that the rate of pneumonia in Ethiopian children with rickets, and therefore a likely vitamin D deficiency, was 13 times as high as in children without that disease. The researchers also point to five studies since the 1930s that have linked reduced risks of infectious disease to dietary supplementation with cod liver oil, a rich source of vitamin D."

http://www.sciencenews.org/articles/20061111/bob9.asp

So sunshine, or cod liver oil, or vitamin D3 supplementation may greatly reduce your risk of becoming sick this winter.