re: How do medical professionals not get sick constantly?

Correct me if I'm wrong, but in order to build antibodies and develop immunity don't you actually have to contract the illness? Ala chicken pox? So that would mean that doctors would get sick quite often if they don't take preventative measures? I mean, the symptoms of being sick (coughing, fever) is actually your immune system responding to whatever bug you have.

Also, like the flu, aren't there different strains of these bugs that mutate and spread every year?