I would have to question a pathologist that would make such a statement after testing only a few animals from one person's collection. Certain pathogens can reek havoc not only on the immune systems, but on the physiology of the animals. To make such a broad statement with very minimal research is irresponsible in my opinion.
Is this based on what the pathologist said, what you have read, or from experience? I feel quite the opposite. I have never had a single issue in regards to health from any pied. I have a very nice group and have friends that would say the same about theirs.