The word influenza comes from medieval latin, influence (of the cold).
Vitamin D boosts the body’s immune system.
We get it (mostly) from the sun.
The sun’s rays kill a virus in 2 minutes.
Less sun exposure in winter, more susceptibility to a virus.
Like a great many other health tips, it’s in my book: www.howtolivethehealthiestlife.com
You can also check out on the Vitamin D council web site the influence of vitamin D. Like vitamin C, it is huge.
There was a map of the US there once (don’t know if it is still there or not) that showed the increased incidence of MS as you went further away from the equator.
P.S. Take vitamin K2 with D3 to prevent hardening of the arteries.