Funny that you should mention that ..
I just picked up two bottles of Vitamin D3, earlier today !
I suffer from SAD (Seasonal Affect Disorder) when at home at upstate New York while downwind from the Great Lakes due to cloudiness and overcast, and restricted sunlight,
or, choose to relocate to Florida for more sunlight inclusion.
I use 'full spectrum lights' (includes blue light- or known as commonly known as 'Grow lights') in a light box while at home, and specialty lights while at while at work,
otherwise I get kinda "funky", "wonky" ,or depressed due to the lack of natural sunlight.
I am third generation of SAD (Seasonal Affect Disorder) who came to Florida, and realized a need for increased sunlight before it actually had the name of SAD.
Most people don't realize that their immunity system is compromised as they get older, or their access to natural sunlight (Vitamin d3) changes with the seasons.
I don't know that the relationship between Vitamin d3, and the immune system dis-function has yet been thoroughly evaluated by modern medicine.
I just know what works for me, and for my mother and grand-mother, that means more access to naturally occurring Vitamin d3 as in Florida, or by pill form Vitamin d3.
The fact that I haven't gotten the flu in several decades, avoid questionable activities, speaks to me, and the need for vitamin d3, either naturally, or, in pill form !
For sure do the D and the special lights.
Funny how we all need that sunlight. Born and raised in Los Angeles where sunny days were boring. For real. Then I lived in Europe where some weeks in winter we didn’t see our shadow on the sidewalk for two weeks, for real. During those periods of no sun, I’d have these vivid dreams I was in the sunshine at a park walking barefoot in the grass. I used to constantly dream about sunshine like my body was begging for it.