One reason could be overexertion of his chakra. In many fanfictions, Naruto might push his chakra limits while using a powerful new jutsu he's created or trying to master. This overextension can cause internal damage, leading to coughing up blood as his body struggles to cope. Another possibility is that he's been poisoned. Some fanfic writers might introduce a new villain who uses a slow - acting poison on Naruto, which gradually weakens him and causes such symptoms.
It could be due to a curse. Maybe in the fanfiction, Naruto unknowingly stumbles upon an ancient curse that starts to affect his health. As the curse takes hold, it begins to damage his internal organs, resulting in him coughing up blood. Also, in some fan stories, he might be suffering from an after - effect of a powerful attack he received during a battle. The attack could have left some lingering damage that surfaces as this serious symptom.
Well, it could be related to some sort of poison. Perhaps he was poisoned by an enemy who managed to slip something into his food or drink. The poison could be slowly corroding his insides and making him cough up blood as a symptom. It might also be part of a plot by a new villain in the fanfiction world who is trying to weaken Harry in a more insidious way than the usual dark wizards.
One reason could be overusing his chakra. In Naruto's world, chakra is like life energy. If he overexerts himself and drains too much chakra, it can cause internal damage, leading to coughing up blood. For example, when he was training intensively or during a tough battle where he pushed his limits.
It could be a new dark wizard. There are always new threats emerging in the fanfiction world. This new dark wizard might have a vendetta against Harry and uses a special spell or potion to make him cough up blood.
It can make him more determined. When Naruto coughs up blood in fanfiction, it often shows that he's facing a serious threat. This could fuel his determination to overcome whatever is causing it, whether it's a new enemy or a personal ailment. He'll be even more driven to prove himself and protect those he cares about.
One reason could be a hidden disease or affliction that is unique to the fanfiction world he is in. It could be something that has been passed down in his family magically. Or perhaps he has been in a situation where he was exposed to a harmful magical energy source for too long. This continuous exposure could gradually damage his body and result in coughing up blood. For instance, if he was trapped near a leaking magical artifact that emits harmful rays.
Another possibility is internal injuries from a battle. Fanfiction often has intense battles that are not part of the original story. Harry could have been hit by a powerful spell during a fight that caused internal damage, which then makes him cough up blood.
Well, it could lead to a lot of self - discovery for Percy. He might start to question his own mortality and his role in the story. This could make him more determined to fight against whatever is causing his condition. His relationships with other characters could also change. Some might become more protective, while others might be afraid to get too close in case they catch whatever is wrong with him.
Perhaps in this fanfiction, Naruto is in a situation where he has no other source of sustenance. He could be trapped in a desolate place with no food or water, and the blood becomes a last - resort option for survival. This would be a very dark and extreme take on the character, as it goes against his usual values.
One reason could be extreme emotional pain. Maybe he witnesses the death of a very close friend or loved one in the fanfiction world. This overwhelming grief could cause such a physical manifestation as crying blood.
It could also be that he is tired of the false expectations created by fanfiction. In fanfiction, his character might be portrayed in ways that don't match who he really is. He wants to be true to himself and his ninja way, so he gives up fanfiction.