@Death_Saved, the author only said there are conditions that only healers can cure, implying that there are likely conditions that may have something to do with the appearance of monsters and/or magic, or the author was simply alluding to untreatable things like AIDS, HIV, Cancer, etc. which modern medicine can't get rid of. In reality, there are no such things as cures, only treatments to help the body fight off disease and sickness naturally and to help reduce symptoms. So author seemed to be alluding to healers being used to deal with extreme diseases and conditions and either supernatural conditions. It also could've been that if the dad was healed for anything earlier, assuming the parasite was smaller and weaker then, that the healing may have gotten rid of the creature also. I'm assuming the dad likely acquired the parasite a long time ago and it sapped his life over time as it grew to the stage it's at now.
As others pointed out, MC only saw the parasite because he has mana vision or whatever and can see mana or mana-based things. We don't know yet if any other unique hunters have a similar skill or if any of them are unique healers and have a similar skill.
@nooneinparticular, My question is why does Kabis assume that if they get stronger they can defeat it? Yet, she also claimed not to know what it was?