I've noticed I've had fewer instances of illness since I started taking vitamin D

In fact, I can count the times I've been seriously ill on one hand, when you take out the times I was sick from black mold in Army buildings.

“For each standard-deviation increase in serum 25-hydroxy-Vitamin D, the risk of a severe outcome was reduced eightfold, and the risk of a critical outcome was reduced 20-fold.”

I also spend a lot more time out doors, which has to help as well.  Fresh air and sunshine do more for your health than any amount of pills.