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.
Well, without more context regarding 'PLL Book 12', it's really difficult to name the 5 novels. It might be that this is a very specific collection within a certain series. Maybe if we knew more about the overall theme of the PLL series, we could make a better guess.
I'm not sure specifically which 'pll' you are referring to. It could be 'Pretty Little Liars'. If so, it's a story full of mystery, secrets, and drama among a group of friends who are being tormented by an anonymous 'A' who knows all their secrets and uses them to play cruel games. There are lots of plot twists and turns as the girls try to figure out who 'A' is while dealing with their own personal issues like relationships and family problems.
It depends on the context and development of the story. Sometimes love alone can overcome many challenges, but in other cases, external factors might play a role.
In the context of 'pll oc fanfic', 'pll' is probably Pretty Little Liars. Fanfic means fan - created fiction. 'oc' stands for original character. So, it's a story written by a fan of Pretty Little Liars that includes their own made - up characters. These original characters could change the dynamics of the story. They might be friends or foes of the existing characters. They could also have their own motives and backstories that add new layers to the already complex world of Pretty Little Liars.