Yes, I believe you do build up an immunity after awhile. I asked my pediatrician the same thing. The one thing I always got when the kids were sick was the GI viruses. I am the kind of person, once I start, I can't keep anything down. I learned fast to remain STRICT NPO for the first 24 hours, then only small amounts of flat ginger ale and dry crackers.
As far as getting sick from your kids, that is hard. You are mommy, and they wipe their boogies on your shouldder when you hug them, LOL, you know what I mean. They need you to cuddle them, and that is how you get sick.
The one thing you can try to do is to wash your hands!!! Wash, wash, wash. Turn your head away from someone who coughs, and teach your kids to cover their mouths when they cough, it is not mean, is is polite as well. Take vitamin C and a multivite.
Each time I ever changed units, or jobs, I would get sick, it was like my body had to get recolonized with the unit/hospital flora. Just try to drink plenty of fluids, and eat healthy, it all helps.
I have a friend who used to swear by taking 20mg of lasix (from stock) 1500mg of vitamin C and drinking water, she swore this "flushed" germs from her body before she had a chance to get a full-fledged cold. And the only sick days she ever took were mental health days