Some people think everything is Yeast. I had a doctor who was absolutely 100% convinced I had a yeast problem. Only thing is, he could not prove it - every test for yeast he could do came up normal/no yeast overgrowth.
This was before I knew I had Lyme, and before this particular doctor learned about Lyme (he's now considered to be an LLMD, at least according to his website).
Yeast is pretty easy to figure out. They can run a long cotton tipped swab up your nose until you can't stand it and rub it around really good. They take it out and rub it on a slide, examine under a microscope, and count the number of yeast cells. Most physicians don't do this, but its an easy way to see where you are with yeast levels.