Every now and again, I hear a story of how a celebrity needed to stop vegetarianism or veganism because it compromised their health. So, maybe. But, I personally don't know anyone. All in all, I would assume out of the majority of people who gain benefits, there will always be an exception.
I have not once been sick since I began a vegetarian diet. Sometimes I wonder though if it's more the crap in factory meat that gets people sick, than if they were eating free range meat.
I think vegetarians have even stronger immune system as we eat much more fruit and vegetables which contain more vitamins than meat does.