Immune super powers

I feel like my immune system must be super strong, because I've been stressing my body a lot lately, and cold season is upon us, but I still haven't gotten sick.  Among other things, I stayed up very late a couple of nights, then donated blood, and then watched the debate at my friend's house while she was fighting off a major nasty cold.  Still not sick.  Of course, just writing this down is probably tempting fate.