My rheumatologist says that no one knows what causes PMR. I'm reading too many posts from people nominating reasons - and taking evasive measures because if this is what did it, well, it's not going to happen again, not if I can avoid it by doing/not doing whatever. We humans really, really want explanations. It doesn't seem logical that a physical condition could impact so many people so suddenly and with such force, and no one knows why. And yet this is what we must accept because our assumptions (and resulting decisions) can do us real harm.