I have always found that a bad hypo, or series of hypos, knocks out my immune system.
It used to be like clockwork: I would hypo (the kind that leaves you dull, aching and fragile for 2-3 days) and then just as i was feeling better, a cold/tummy bug/sore throat would strike. I always assumed that my immunity dipped during the hypo and the virus percolated for those 2-3 days, then emerged.
Until i found this forum, and started reading here, i had no proper appreciation that high blood sugar was the cause of diabetic complications. All my experience had been with low bg, not high - and that was bad enough. I had a severe bout of despair when i realised that keeping bg UP wasn't enough. I had to keep it DOWN too. Soul destroying, really. Of course, now i know better. The control gives so many benefits i celebrate it.
Incidentally, i had a bad chest infection, around the time i think i slipped into T2. It was another good motivation to sort things out. Since i have kept my bg controlled (up and down), i have had fewer colds than my colleagues at work.