The difference between Vitamin D, and Vitamin D-3 is the source. Basic Vitamin D usually derives from soy. In order to avoid soy, Vitamin D-3 comes from better sources, such as Lanolin, which is sheep's wool, so it is still a vegan source.
Vitamin D is the world's single most common vitamin deficiency and an estimated 85% of people are deficient in the US. We are supposed the get Vitamin D from the sun, but not many of us spend the time in the sun needed. Then there is the fact that too much sun exposure is not safe due to the ozone layer being depleted. Therefore we need to take a Vitamin D supplement especially during the signs and symptoms of a cold or flu.
Vitamin D helps the immune system to fight infections. With a near-optimum blood level of vitamin D, you can expect fewer colds and flu.
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.