There could be a curse placed on him by an old enemy of his family. This enemy has been waiting for the right moment to strike. They manage to get close to Harry and put a powerful curse on him that causes his injuries. It could be a curse that slowly drains his magic and weakens his body at the same time.
Maybe it's a result of a dangerous magical experiment gone wrong. Harry might be trying to test a new spell or a new form of magic that he read about in some old, mysterious book. But he doesn't fully understand it, and it backfires on him, leaving him badly injured.
It could be Dumbledore. If he is too focused on his grand plans against Voldemort, he might overlook Harry's well - being and needs, which could lead to Harry's death from neglect.
There's also a possibility that it was an accident. Maybe Harry was exploring some ancient and dangerous magic, and something went wrong. A spell backfired and instead of achieving what he intended, it put a strain on his heart and led to the heart attack.
It could be a rogue Death Eater who has been hiding and waiting for the right moment to strike at Harry. They might use some forbidden dark magic to seriously harm him.
It could be a rogue Death Eater who managed to catch Harry off - guard. Maybe they were lying in wait and launched a surprise attack when Harry was alone, away from the safety of Hogwarts.
It could be Danzo. He has always been a bit of a shadowy figure with his own motives in the Naruto series. He might see Naruto as a threat to his power and influence within the village and engineer a situation where Naruto is executed.
It could be the students. They might think Snape is too gloomy and old - fashioned in his appearance. So, as a bit of mischief or maybe out of kindness, they use their magic skills to give him a makeover. For example, Harry Potter and his friends could be behind it, using spells they learned in their Defense Against the Dark Arts class.
It could be Malfoy. He often has a vendetta against Harry and might have found a way to make him collapse during potions class, either by tampering with his ingredients or distracting him in a way that leads to a mistake in the potion - making process.
It could be Death Eaters. They might have finally found a way to capture Harry without alerting Dumbledore. His 16th birthday could be the perfect opportunity as everyone might be a bit more relaxed thinking he's not quite of age yet for the full - on final battle.
It could be the wizarding authorities. If they discover that Vernon has been abusing Harry Potter all these years, they might decide to arrest him for child abuse. Even though they are in the wizarding world and Vernon is a Muggle, they could still have some jurisdiction over the well - being of a young wizard like Harry.
It could be a case of betrayal from within. Maybe someone who was thought to be an ally, like a wizard who was secretly working for the dark side all along. They could have used a powerful and unknown curse to kill Harry in the middle of the night without anyone suspecting them at first.