Vitamin D helps our body use calcium and phosphorus. That's why many calcium supplements have vitamin D added. It's like an aid in helping our bones get the calcium. Like that commercial were the key is in the shape of a D and they tell you that vitamin D helps unlock calcium to help build strong bones.
Children that don't get enough vitamin D end up with Rickets. Where the bones don't grow right causing deformities. This also leads to weak bones and weak muscles in their adult years.
So research has shown that it also may help conditions like diabetes, heart conditions, and multiple sclerosis. I know I thought "sure that is what they thought at one time about vitamin C". But the fact that they find many more cases in the northern hemisphere makes one think maybe it does help. Not cure but help. Then I learned that some conditions like Crohn's disease, cystic fibrosis, and celiac disease can't absorb vitamin D from foo.
So really there isn't the proof of Vitamin D curing anything. It just studies that show it helps. So it can't hurt to take a supplement. Or get outside and enjoy a day in the sunshine........ Yes you can overdose on vitamin D but only if you take more than 10,000 IU's per day for long periods of time. And never from it natural state as in sunshine or foods. So a little can't hurt but may help.