Cold, flu or other respiratory infections you can basically counter with vitamin D.
Dose 20-30k IU and most of the time flu or colds are gone overnight (at least for me). And yes, flu is gone overnight e.g. 1 day... whereas most people infected by the same one take 7 days. It's pretty amazing
Sleep is the other big one obviously. Sleep a lot, vitamin D. Eat as much as you feel comfortable with.