Prior to and during infection it is wise to take vitamin D supplements. The reason why is because Vitamin D SLOWS DOWN a cytocline storm (a monster immune response that attacks the body indescriminatly)

It also speeds up safe healing. Perhaps you see the connection between dark skin people having less vit D production compared to light skinned people and as a result have a higher rate of morbidity and fatality. There are many other factors and influences but vit D is readily available and relatively safe.

Getting sufficient sunshine is even better than supplements.

I heard it on npr