Maybe he was poisoned. There are many powerful poisons in the Naruto universe. If an enemy managed to poison Naruto, it could affect his internal organs and make him cough up blood. Just like Sasuke was once poisoned, Naruto could also fall victim to such a tactic.
It could also be due to a curse or some sort of dark jutsu. There are various evil jutsus and curses in the series. If Naruto was targeted by one of these, it might cause him to have internal bleeding, resulting in coughing up blood.
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.
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.
It could be due to a dark curse or spell. Maybe he was attacked by a powerful dark wizard using a forbidden spell that affects his internal organs and causes bleeding.
Maybe he was poisoned. There are many villains in the Naruto world who use poison. If Naruto was poisoned during a fight or an ambush, it could lead to him coughing blood as his body tries to fight off the toxins. Just like how Sasuke was once poisoned and had to be treated.
Well, it could be a result of a dark spell cast on him by a Death Eater. Maybe they found a new and insidious way to harm him without leaving an obvious trace.
Coughing up blood referred to coughing up blood from certain parts of the respiratory tract in the lungs. Common causes include chronic asthma, dilated bronchi, respiratory diseases, and cardiovascular diseases. The symptoms of hemoptysis may be bright red or pink blood, which may contain mucus. The amount of hemoptysis is usually small, but if there is a large amount or frequent hemoptysis, you should seek medical attention to determine the cause and receive appropriate treatment.
It could be a dark magic curse. Maybe some enemy has cast a spell on Harry that is slowly sapping his health, leading to him coughing blood.
It could be a sign of a major turning point. If Naruto starts coughing up blood, it might signal that he's in a really dangerous situation and needs to find a way to get out of it fast. This can lead to new adventures as he searches for a cure or a way to regain his strength.
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 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.