I'm with LV and Brahma, maybe once or twice a year. Was true in fat, lazy smoker mode, and still true in slightly less fat, healthy, nonsmoker mode. When it happens though, it's like playing catchup for all the times I didn't get sick. I think exposure has a lot to do with it too.
For instance, the sickest I think I ever was, was when I joined the army. Most people get sick when they 1st arrive for inprocessing; they call it the GI Crud. Something about being shoved into a barracks lifestyle with people from all over the country, so you get varients of stuff you would never normally be exposed to.