Airline workers

I have a friend who has flown for 30+ years, international as well as domestic flights. She was diagnosed a couple of years ago with CIDP, and her doctors told her that there was an increased number of patients with CIDP among airline workers. So who knows? Bad air? Exposure to multiple viruses a/or bacteria? Stress? And then there was that case a year or two ago about the workers in a slaughterhouse that used a high-pressure air hose to blow pig brains out of the skull, and almost all of them came down with CIDP. Who knows?