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.
Perhaps in the fanfiction, Ponyboy got sick because he overexerted himself. He might have been running around a lot, getting into fights, and not getting enough rest. His body just couldn't keep up anymore. And on top of that, he could have been exposed to some kind of virus or bacteria in the dirty and crowded areas where he and his gang hang out.
The other characters' reactions could vary. Some of the more hot - headed gang members might initially be impatient about Ponyboy being sick and not being able to participate in their activities. But as they see how serious it is, they would come around. The Socs, if they knew, might not care much on the surface, but there could be a few who feel a bit of sympathy, especially if they start to see Ponyboy as more than just an 'outsider'. And the girls in the story, if there are any, would probably be very motherly and bring things like soup and warm blankets for Ponyboy.
One possible reason could be that he's a Greaser. The Socs might see him as an easy target just because of the group he belongs to. They might want to show their dominance over the Greasers.
His friends might be able to save him. If they notice his disappearance quickly enough, they could band together and search for him. Maybe they have some knowledge or items that can be used against the demon.
In the fanfiction, one possible cause could be that he ate some bad leaves. Maybe the leaves were contaminated with some kind of toxin from a nearby factory. Another cause could be that he caught a strange virus from an animal that recently visited the zoo.
Perhaps he was poisoned. Given the dangerous situations he often gets into, someone could have slipped a poison into his food or drink. It might be a poison that is designed to specifically target demigods like him. Or, it could be related to his heritage. Since he is a demigod, there could be some genetic factor that makes him more susceptible to certain types of sickness, and in the fanfiction, this could be explored as he gets sick.
Well, in the Chronicles of Narnia, Lucy could get sick perhaps due to the magic of Narnia going awry. Maybe she was exposed to some dark magic or a spell that made her ill. Another possibility is that she ate something in Narnia that her human body couldn't tolerate. For example, some strange Narnian fruits or plants that had an unexpected effect on her.
It could be a rival gang. They might see Ponyboy as a threat or a way to get back at his own gang. Another possibility is a deranged fan of his, someone who has an unhealthy obsession and wants to keep him all to themselves. Or it could be a group of criminals who mistake Ponyboy for someone else who has something they want.
One possible plotline could be that Ponyboy and Sodapop get caught in a sudden storm while out in the open. They're both scared as lightning strikes close by. They huddle together in an abandoned building, sharing their fears and memories from their tough lives. This situation could strengthen their bond and lead to some deep conversations about their dreams and hopes for the future.
In the 'ponyboy gets taken by a demon' fanfiction, a lot can unfold. First, the demon might have been observing ponyboy for a while, waiting for the right moment to strike. Once taken, ponyboy could be exposed to all kinds of strange and terrifying things in the demon's world. He might meet other captives or creatures there. He would have to rely on his wits and any latent courage he has. There could also be a chance that he discovers some sort of connection between himself and the demon's world that he never knew before, which could either help him escape or make his situation even more complicated.
Well, a great plot twist might be that Ponyboy discovers the kidnappers are actually part of a secret society that has been observing him for a special reason. Maybe they think he has some hidden power or knowledge. Also, it could be that during the kidnapping, Ponyboy finds out that there is a traitor among his own group who helped the kidnappers. This would add a whole new layer of drama and mystery to the fanfiction.