It's very possible that you may get ill several times in the beginning of your career as you are exposed to new bugs.  You will build immunity over time especially if you work in an environment in which you are exposed to a multitude of germs.  When I started in the er 15 years ago, I was sick,  vomiting, diarrhea...etc, twice in the first 3 months.  Since then, I typically don't get much more than maybe a seasonal allergy type of syndrome that is short-lived.  My wife and kids get sick several times each year, but me...nothing.  I'm probably the carrier!