I never really noticed a problem with colds/flu bugs, but that's probably because I rarely left the house for 18 months!
I did have...er...intestinal issues for the first few months I was here, until I convinced my MIL that, yes, when they print 'Refrigerate after opening' on the side of the jar, they really mean it.
I did start a job in November, where I'm in and out of schools, seniors' centres, etc. a lot, and so for the first time in my life, I got a flu shot. I regretted that for about 10 days, but I haven't really been ill since. So I'll probably get one again next year.
Really, though, I think the only thing to do is take the usual precautions. Avoid touching things, wash your hands frequently, carry hand sanitizer gel. There are a couple of products available that are basically a gel that you coat the inside of your nose with, and they're meant to stop you catching colds, but I'm personally skeptical. Same with pretty much any supplements/homeopathic stuff. Even vitamin C, which is somewhat helpful-- your body can only actually absorb a certain amount of it, so must of what's in those 'mega-dose' products (stuff like Emergen-C) goes right through you, giving you really expensive urine, and no more health benefit than you'd have gotten from eating an orange.