This is true.
I think some of it has to do with the weather turning cold, people who are already compromised catching something that is going around during a holiday gathering, compounded by eating and drinking too much of the wrong stuff, compounded by getting off one’s medication and vitamin schedule, compounded by getting off one’s sleep schedule when one is staying somewhere else or people staying with them, keeping them up, sadness that a family member or good friend is no longer at holiday gatherings, stress caused by family fights...
I could go on.
Yep probably all of that.