According to a 2009 article in the Annals of Epidemiology it stated - "Supplimentiing with vitamin D3 has the potential to reduce cancer deaths in America by 75%"

Our bodies need Vitamin D to help boost our immunity. Vitamin D3 has also been shown to prevent many strains of the Flu, including the Swine Flu. It also plays a role in cancer prevention. Most Americans tend to be Vitamin D deficient even in areas as sunny as Colorado. Your body will make Vitamin D when exposed to the sun however the need to use sunscreen to block the sun's damaging rays also blocks the production of vitamin production so we find that supplementation may be required.

Have your vitamin D levels checked at least once per year and supplement (if needed) with a high quality D3 (liquid or sub-lingual is best).