I have read that it's bad if you don't ever get colds. Colds are elimination techniques that are good and mean you're strong. Yet, the other camp says that it's weak immunity that allows you to get a cold. Dr. Stoll brings these up in his book. One is something as simple as air humidity.
Exposure is another obvious variable.
I used to consult for a group of 6 Ear, Nose and Throat specialists. Whenever they would hire someone new they would find that new hire would have problems for the first 3-6 months with picking up whatever bug was going around at the time. If your adrenals were not functioning you'd be dead. Every organ and organ system in our body is made to accomodate short term stress by boulstering it's own function. The problem is that if it's forced to carry that stress for too long it will ultimately become exhausted.