Since my original post in this thread about illnesses being rare, I did think of one interesting counterexample that weakens my theory.

In the Stormblood White Mage storyline, the mother of the prodigal Padjal at the center of the storyline is suffering from some kind of wasting disease. As it turns out, she was also host to a particularly vile Voidsent, which may or may not have been causing the disease - but whether it was or not, no one really seemed to think of her disease as anything UNUSUAL. The vibe was more, tragic, but all-too common.

I think they did mention that she'd be better off under the care of the Conjurers of Gridania (something she refused, as she rightly assumed the Padjal would take her daughter away), but I don't think it was ever stated that they would for certain be able to cure her.

Anyway, just something else to chew on.