Vitamin D deficiency (low levels of vitamin D in the blood) is a common problem. A recently discovered protein, called hCAP18, likely plays an important role in the immune system and may depend on adequate levels of vitamin D. It is not known what levels of vitamin D are needed to allow the body to make this protein. Nor is it known if giving vitamin D to people who are found to be deficient will help boost levels of hCAP18. This study aims to clarify the relationship between vitamin D levels and hCAP18.