@Kshatria,
While I feel for the most part MC has been an idiot, impulsive at times, and bullheaded when Kabris gave him advice throughout this story, I was just saying that someone said he was being impulsive by not knowing everything regarding this sickness and wanting to heal his father long before this reveal. I'm not necessarily saying he should've known that this exact thing would happen. Earlier on, MC did mention there are diseases or sicknesses that modern medicine doesn't know about indicating that there was obviously something strange about his father's condition. So, I think the person had a point about putting mana in his father might potentially make things worse without actually knowing anything. And here we are, I was just pointing out that person called it. Either they had knowledge of the story from elsewhere like a novel, or they just predicted it properly.
Also, they actually saw the parasite when Kabris used her eyesight ability or whatever to look at mana, and then they saw it. They both could've literally done that prior to him healing his father to see if they could learn anything more about his condition because again, the concept that some diseases and sicknesses that have arisen after the monsters invaded the world is there. So a thinking person might investigate a little further before acting. Again, I'm not necessarily blaming MC for this one, but his track record of idiocy IMHO hasn't helped him for me either.