Vitamin D helps the immune system to fight infections. With a near-optimum blood level of vitamin D, you can expect fewer colds, 'flu, and other unwelcome opportunists (including Swineflu).
Not only does Vitamin D enhance your immunity, but it inhibits the development of destructive auto-immune diseases such as diabetes, rheumatoid arthritis, lupus and multiple sclerosis.
Vitamin D is a major inhibitor of cancer, and part of this effect may be attributed to the increased vigilance of the immune system.