Another possibility is that one of the other characters in PLL has a hidden motive. For instance, someone who is jealous of Allison's popularity or someone who she had an argument with in the past. They might have found a way to make her sick without being suspected at first.
Well, it could be about Allison's struggle with a mystery illness that may or may not be related to the main PLL plot. Maybe she starts getting sick and her friends suspect it's something more sinister, like someone is poisoning her to keep her quiet about a secret.
Maybe it could be about how Caleb's sickness affects his relationships in the Pretty Little Liars world. For example, how his friends and loved ones react to his illness, and how he copes with it himself while dealing with all the secrets and drama in Rosewood.
The plot might revolve around Aria getting sick as a result of all the stress from the Pretty Little Liars' chaos. Her sickness could be a symbol of how the lies and secrets are taking a toll on her. In the fanfic, we might see her struggle with her illness while also trying to figure out who 'A' is. It could be that her weakened state makes her more vulnerable to 'A's attacks or makes her see things in a different light.
Maybe it's about how Caleb's sickness affects his relationships in the PLL world. For example, how his friends and loved ones react to his illness and take care of him.
In the PLL fanfic, when Caleb gets sick, it could lead to a lot of worry among his friends. They might try to take care of him, like Hanna bringing him soup and making sure he takes his medicine. His illness could also be a distraction from the main mysteries in Rosewood, causing the group to focus more on his recovery for a while.
The story would change in several ways. First, the focus would shift a bit to Caleb's health. His friends would be preoccupied with taking care of him. Second, it might slow down the investigation into the usual PLL mysteries as they are distracted. Third, his relationship with Hanna could be put to the test.
It could make Ezra very protective. He might put aside some of his own problems to focus on taking care of Aria. Her friends like Spencer, Hanna, and Emily would also be worried. They might visit her often, bringing schoolwork and gossip to keep her in the loop. But at the same time, they could be distracted from the 'A' mystery, which might put them at risk.
Hanna could be one of the main characters. Since she and Caleb are in a relationship, she would probably be very involved in taking care of him and trying to find a way to make him better.
Hanna would likely be a main character. Since she and Caleb are in a relationship in PLL, she would be deeply affected by his sickness. She would probably be by his side, trying to take care of him and also trying to figure out what's wrong.
In this fanfiction, it might be due to his poor living conditions. He doesn't always have the best food or a warm, clean place to stay. His body could be weakened from not getting enough nutrition, which makes him more likely to get sick. Also, the emotional turmoil he goes through with losing friends and dealing with the Socs could take a toll on his immune system.