I actually feel like my immune system is better than most. I credit this to my constant exposure to the walking germs in combination with the vitamin cocktail I take every evening with my dinner. Without the vitamin supplements, I fall ill rather quickly.
I also suffer from food allergies that are very severe. Fortunately, it's only to two types of tree nuts (so far) and I can usually avoid exposure. Occasionally, I will get accidentally exposed. Once recovering from the anaphylactic shock, my immune system is weak for about 5 to 7 days.
When I start feeling a little sniffly, I take extra vitamin C for a few days and can usually thwart whatever is attacking me.