Man, I had a bout of swine flu last week - it was awful. I felt like death warmed over for four days, and had a miserable Christmas. Of course, I didn't get a flu shot, even though the Canadian health system is practically begging you to go. I regret not doing that now.
But in general, I think Canadians and Americans have pussified their immune systems. When I visit the Philippines, I have to avoid raw vegetables, tap water, and ice. And that's even though I always bring a good supply of Immodium for when the eventual bout of deadly diarrhea hits. But the natives eat anything, anywhere, and never get sick. Clearly, their bodies have stronger or more complex immune systems than people from Canada/the US. And you can lose that strength; my wife came to Canada from the Philippines more than 30 years ago, and when she goes back now, she has to watch what she eats as well.
It's funny; my grandmother used to tell me "A little dirt never hurt anyone" if I dropped an apple on the ground, but my mother was a fanatic about having an almost sterile cooking environment, and wouldn't shop at some grocery stores because they were "too dirty". Unintended consequences?