I don't know if they have a hard percentage number, but it is very common. Scientist for years have thought all the auto-immune diseases are linked to celiac plus many other things. They have even linked schizophrenia with celiac. A interesting book to read that talks about this is called Dangerous Grains, can't remember the author right now. My mom is borrowing my copy.
I personally have asthma, hypoglycemia plus fibromyalgia.