He probably uses pretending as an excuse so others wouldn't be as worried but it doesn't work very well
He have an illness.. remember the blood cult kid said his mana is very bright but broken/fractured. But he's also powerful martial artist (ex general, emperor in past live). Just like in real live sometimes there are athletic, active people who surprisingly have manageable heart disease.
imo he *does* have a disease, but exaggerates the symptoms to outsiders (like he's on the verge of death so the opponents will think they have the opportunity to strike while it's hot)
in actuality, while it *is* serious, he isn't going to drop dead at any given moment (at least i hope he isn't)
hey, im a little confused. the mc does have an illness right? why does he feel the need to pretend when in fact, he DOES have it