I can't believe no one mentioned the disease themselve by this point.
So we have a 16 wis ritual caster of level 6, trained in heal and with no special bonus (Can't say I'm min/maxing, here). He has +11 to heal. Minimum result 12.
Mummy Rot is Level 11. My example can't kill you while trying to heal mummy rot! Any disease that could kill you if this guy was your doctor would have to be worse. Something like Mindfire (level 16) or Hellfever (level 21).
Bottom line, cure disease might kill you only if you are afflicted by some really high fantasy version of the ebola!