This is an excellent article in an excellent publication (Science News). It catalogs the current controversy about the amount of D we need in our diets (or from sun exposure) and lists the huge number of bodily functions that it is critical for. Here is just a sampling. Try to read the whole article if you're at all interested. I guarantee you will be amazed. The infection fighting ability was discovered inadvertently a century ago when doctors treated tuberculosis patients with rest in the sun. Except no one understood why the cure worked. Now we do.