I take D3 twice a day. Single dose is 2000 I think. No cold for years, but if I think there is any sign, then I add Coldeze.
I got a fever and a cough about 1 march last year even though I used the D3 daily. I thought, “this is odd.” I took the D3 at 900 IU per pound of body weight and was running through symptoms gently of what I later learned was probably he CCP bug. It lasted most of the day and went away. Were it not for antibody tests I would have remained mystified. The mega dose is what I started the D3 with two days into a flu 16 years ago that had people miserable for 3-6 weeks. That D3 which I took on the advice of my mother-in-law had it fixed overnight and made me a believer.