Often, his friends would rush to his aid. They might try to find a cure or figure out what's wrong with him. Maybe Hermione would start researching in the library right away.
In some fanfics, it could lead to a journey for a special healer or a magical artifact that can cure him. Harry's situation would be the catalyst for an adventure. His friends would accompany him, and they would face many challenges on their way to find this cure or artifact. This journey could also be a time for character development, as they deal with the fear and uncertainty of Harry's condition and grow stronger in the face of difficulties.
The plot could develop in a way that it reveals a deeper conspiracy. His condition might be part of a larger plan by the dark forces. So, it would lead to a search for the real culprits behind his illness. His friends and allies would then start to investigate, following leads and facing various obstacles along the way. They might encounter new characters who are either helpful or obstructive in their search for the truth.
After he throws up, he might discover a new clue. For example, the vomit could have some strange color or texture that indicates what kind of poison or magic is affecting him. Then he could start looking for a cure.
The other characters could rally around him to find a cure. They might search for ancient spells or potions to heal him.
Often, it starts with the incident that causes the concussion. Then it shows how Harry's daily life is disrupted. For example, he may struggle in his classes or have difficulty with his Quidditch training.
Often, it starts with Harry's decision to become a doctor. Maybe he realizes that there are so many people, both Muggles and wizards, in need of help. Then, he begins his studies. If it's in the wizarding world, his studies might include both magical and non - magical medical knowledge.
Often, the betrayal is sudden. For example, if it's his friends who betray him, it might start with a small incident that Harry doesn't think much of at first. But then it snowballs into a full - blown situation where his friends turn their backs on him completely. Then Harry has to struggle to deal with this new situation while also facing other threats in the wizarding world.
Often, it starts with an event that causes the misperception. For instance, Harry might perform a spell that has an unexpected and overly powerful effect. Then, the plot develops as others react to his perceived divinity. Some may try to use him, while others might be genuinely in awe.
Often, it starts with the event that causes Harry's brain damage. Then, the plot might develop around how he has to relearn magic or deal with his changed mental abilities. For example, he may struggle with spells he used to be able to do easily.
It could lead to a discovery of a hidden plot against Harry. His coughing up blood might be a sign that there is a much bigger conspiracy at play, perhaps involving someone close to him who wants to weaken him for some ulterior motives.
Often, it starts by establishing the character with the weakened heart, like Harry. Then it shows how this condition affects his daily life at Hogwarts, such as struggling to keep up in Quidditch or during difficult magic classes.