[Xietor]

most diseases are killed with an increase in temperature. But maybe there are special diseases that inflict lava creatures. But if you get too thematic and they are poison and disease resistant, they may lean towards unbalanced.

Though it could give them a huge edge over a couple of races that may use poison(Sauromatia). Or water race troops with poison spears etc.

And being slow moving it would take poison cloud/dragon breath out of the already limited arsenal of the nature races. Ctis uses Foul Vapors.