Two things -


I think that there is something to the idea that some people may be less susceptible, have stronger immune systems and so on.


Also, I'm sorry to burst anyone's bubble, but I just don't think that they are all that easy to catch because they are not airborne. Think how many colds you've had versus stomach viruses. I bet there's quite a difference. They are not as easy to catch as our anxiety would try to have us believe, nor are they as ubiquitous as our anxiety would have us believe.