Some of the Pokémon he has helped in the past might also come to visit. For example, the Pidgeot he released might come back with some special herbs that it found in the forest. And the wild Pokémon in the area where he is staying might also bring small gifts or items that they think could help Ash get better, out of gratitude for all the good things Ash has done for Pokémon in general.
Professor Oak could also come to visit Ash. He might bring some rare Pokémon medicines from his lab. And of course, Ash's mom would be there. She would cook all of Ash's favorite foods and make sure he takes his medicine on time and gets enough rest.
His friends like Misty and Brock would probably visit him.
Pikachu could be a big help. Since Pikachu is Ash's closest Pokémon partner, it would stay by his side constantly. It could use its electric powers in a gentle way to warm Ash up if he has a fever, like a little electric blanket. And just having Pikachu there would surely lift Ash's spirits.
Probably Gloria, the hippo. She's kind - hearted and might stay by Melman's side, bringing him water and making sure he's comfortable. She could also use her size to protect him while he's weak. And there's always Marty, the zebra. He's energetic and could run around to get help from the zookeepers or find any other resources needed to make Melman better.
Obviously, her best friends Sam and Freddie would visit her. They are always there for each other. Maybe Gibby would also come over with his unique brand of humor to try and cheer Carly up.
Maybe it's a legendary Pokemon in disguise. In some fanfictions, legendary Pokemon often play important roles. It could give Ash the egg as a test or because it senses that Ash has a pure heart. Another possibility is that it could be a young trainer who can't take care of the egg anymore and trusts Ash to do a good job.
Seth could be the one to help her. As her brother, he would be very concerned and might use his own knowledge of Fablehaven and its secrets to try and find a cure. He could also enlist the help of some of the friendly magical creatures they've befriended, like the fairies or the satyrs.
Anna would surely be the first to help Elsa. Anna has always been there for her sister, and when Elsa gets sick, she would do everything she could, like bringing her warm soups and staying by her side to keep her company.
His family members would likely help him. For example, his parents, Hank and Barb, have a lot of experience with superpowers and could try to figure out what's wrong.
Her best friend Veronica would probably be one of the first to help. Veronica is wealthy and could use her resources to get the best doctors for Betty.
One possible plotline could be that while Ash is sick, his Pokémon have to take care of him. Pikachu might gather berries to make a special potion to help Ash get better. Another could be that Ash's friends are on a journey without him because he's too sick to travel, and they constantly worry about him and send back items to help his recovery.