Well, there's 'Harry's Malady'. It's a fanfic that focuses on a magical sickness that only affects those with a special type of magic, like Harry. It shows how isolated he feels at first, but then how the wizarding community comes together to try and help. And 'The Sickened Seeker' is a good one too. In this story, Harry's sickness is related to his Quidditch skills. It's a unique take on a sick Harry Potter fanfic as it combines his love for the sport with the struggle of dealing with an illness that affects his performance.
There are often 'Harry Potter sick fanfic' that focus on Draco Malfoy being sick. Maybe he's been poisoned by a dark spell in a duel gone wrong. His Slytherin friends try to keep it a secret while they look for help, but some Gryffindors find out and there's an unexpected alliance as they all work together to save him. It shows a different side of the Hogwarts houses' relationships.
One could be 'The Sickly Days at Hogwarts'. In this fanfiction, Harry is sick with a mysterious magical illness that affects his magic. The story focuses on how his friends Hermione and Ron try to find a cure while also dealing with the fear that Harry might not recover. Another is 'When Harry Potter Took Ill'. Here, the story shows how the whole Hogwarts community reacts to Harry being sick, from the teachers trying to figure out what's wrong to the other students showing their concern.
There could be a fanfic where Artemis is sick and Harry Potter uses his magic to try to heal him. Maybe he brings some potions from Hogwarts. For example, the Pepperup Potion which is known for curing common ailments. Harry might face challenges in using magic outside of Hogwarts to help Artemis, as it's against wizarding laws in some cases.
A really interesting 'Harry Styles sick fanfic' might be set during his tour. A fan gets sick while waiting in line for his concert. Harry finds out and arranges for special medical attention for the fan, creating a story full of his generosity.
There's a fanfic called 'Sickly Harry's Struggles'. In this story, Harry gets a strange sickness that affects his magic. It's interesting as it delves into how he tries to cope with it while still facing the challenges at Hogwarts. It also shows the different potions and spells the school tries to use to heal him.
Another option is Wattpad. Although it has a wide range of content, there are likely some 'Harry Potter sick fanfic' there as well. You might have to do a bit more digging compared to the other two sites, but it's still worth a try. You can start by searching for 'Harry Potter' and then skim through the results for stories that involve sickness.
In a fanfic where Fred is sick, it might start with him collapsing during a Quidditch match. Everyone is shocked. The Weasley family rushes to his side. Ginny is especially distraught. They take him to St. Mungo's, and there they discover it's a very rare wizarding disease that is passed down through certain wizarding bloodlines. The Weasleys start to recall if there's any history of such an illness in their family. While Fred is in the hospital, he has strange dreams about the Marauders and their time at Hogwarts, which might give him some clues on how to get better.
Some 'Harry Potter sick fanfiction' involves the spread of a magical epidemic within Hogwarts. This can lead to interesting plotlines such as the school being quarantined, and students having to rely on their own knowledge of magic and teamwork to survive and find a cure.
One fanfiction could be where Harry catches a magical illness that only affects wizards. He might have strange symptoms like his magic going haywire. For example, his spells could backfire in unexpected ways, causing chaos at Hogwarts.
One common theme is the idea of friendship. When Harry is sick in fanfictions, his friends like Hermione and Ron often come to his aid. Another theme is the mystery of the illness. It could be a new dark magic - induced sickness that no one has seen before.