Basically, if you have kids, you're fucked, immunologically. It's near impossible to get enough sleep and still get everything done, and if they're in day care or school, they're going to bring home every bug known to man (and some that aren't)

Plus, in my case anyway, I find it near impossible to get regular exercise and eat healthy at the same time due to the time and budget constraints, (and I live in a tourist town), so I'm just resigned to being sick about once per month.

Basically, like you said, avoid kids and definitely don't have kids if you want to dodge colds and flu. Or if you do have kids, be independently wealthy so you can afford to take time to sleep, exercise, and eat healthy.