Vitamin D is essential for a healthy immune system. In fact, a sign of vitamin D deficiency is getting sick often. Some evidence suggests vitamin D supplements might help reduce your risk of developing a cold or flu. In addition to this, vitamin D has also been linked to the prevention and protection against autoimmune diseases.
Note: If you have sarcoidosis (an inflammatory disease) or a condition that affects your parathyroid glands (these regulate calcium in your body), it can make it difficult to interpret your vitamin D results. So in this case, it's best to work with your doctor and get specialist input.