I started escorting in late 2013. That winter I got lots of tummy bugs. I had the shits at least three time - I think it was norovirus (I am one of those people who never vomits so I didn't recognise it at first.) I think I had a flu, as well. But this past winter went better. The only trouble I have had is my eyes - I have had dry, irritated eyes and that made them susceptible to conjunctivitis. But I've had no tummy bugs and never even caught any of the flus that were going around.
There may be a bit of a gauntlet to run and then you get immune to a lot of things? I know when I went into the military, everyone got ill during basic training. There's actually a specific flu virus that lives at that particular training command that has been studied by medical researchers because the whole place is like a petri dish. Pink eye also ran rampant. Everyone was ill for about four weeks and then in the fifth week you'd feel better. There are plenty of other commands where enlistees live in close quarters but something about the training command seemed unique. So, maybe you really will get better and stay mostly better.
Anyway, one thing I do is to take a lot of Vitamin D. And Omega 3 capsules. No doubt there are other immunity-boosting things out there but I haven't really looked into any of them.