Here is a video that is worth your while to watch:
I used to live in the USA and took part in the study for Grassroots but it turns out that I was an outlier! That means that it got me even thought my vitamin D 3 levels had been adequate for several years.
That said I had treatment and hope that I had an easier time of it in terms of recovery/recurrance than if my levels were low.
You can get high dose vitamin D3 on Amazon: