ELI5:

Your body uses vitamin D to tell different parts of your body what to do. Some people have a problem. The body stops listening to vitamin D. So those parts of the body don't work properly and the people get sick. They get "auto-immune" sickness, like psoriasis or MS.

A doctor can test your blood and look for this problem. The article shows how to do that test. It teaches doctors a lot about the problem.

If a person takes a lot of vitamin D, they will get very sick. But if their body isn't listening to vitamin D, then they won't. So a doctor gives you vitamin D. And she checks your blood every week to see if it's enough or too much. This is called the "Coimbra Protocol". Some people get healed with it.