Well, Jason would be right by Leo's side, offering his support and strength. Piper would use her charm - speak to try and make Leo feel better, maybe to get him to eat or drink if he was too weak. Frank would transform into different animals to get things for Leo, like medicine or a warm blanket. They all care deeply about Leo, so they would do everything they could to help him get better, from taking turns staying with him to going on quests to find a cure if necessary.
His friends would most likely be very worried. They would gather around him, trying to figure out what was wrong. Percy might use his water powers to cool Leo down if he had a fever. Annabeth would be researching in the library, looking for any information on demigod illnesses. Hazel could use her powers to sense if there was any dark magic involved in Leo's sickness.
His friends' reactions would be a mix of concern and action. They are used to facing challenges together, and Leo being sick is just another one. They would start by making sure he was comfortable, like getting him a proper place to rest. Then, they would pool their resources and knowledge. Grover might use his nature - related abilities to find some herbs that could help. They would also keep Leo's spirits up, sharing stories and memories to give him the will to fight the sickness.
In most 'Naruto gets sick' fanfics, his friends are very concerned. For example, Sakura will immediately start using her medical skills to diagnose and treat him. Sasuke, although he may not show it outwardly as much, will stay by Naruto's side and keep a watchful eye. Shikamaru might come up with plans to make Naruto more comfortable during his illness. And Hinata, she would be really worried and would probably bring Naruto some home - cooked food to help him regain his strength.
Well, Betty will be super worried. She's always had a soft spot for Archie. She might skip her own activities to stay by his side, constantly checking his temperature and making sure he's comfortable. Jughead, on the other hand, will try to keep things light. He'll tell Archie stories about the latest strange things he's seen in Riverdale to distract him from his illness. Veronica will be more of the practical one. She'll call in doctors or specialists, using her family's influence to ensure Archie gets the best care possible. All in all, his friends will rally around him.
His friends would most likely be very concerned. They would probably bring him things like food and medicine. TK might pray for his quick recovery.
His friends would be devastated. Scott, for example, might go into a state of extreme anger and blame himself for not being able to protect Stiles. Lydia could be inconsolable, using her banshee powers to try and find some connection to Stiles in the afterlife.
Pikachu would be very worried. It would stay by Ash's side constantly, using its electric shocks to keep him warm if he has a fever. His other Pokémon like Bulbasaur might use its vines to bring water or herbs to help soothe Ash's symptoms. Charmander could use its fire to keep the area around Ash cozy.
I don't think the gods read Leo Valdez fanfiction. Gods in that universe are often busy with more important things. They have to manage their powers, deal with other gods, and look after the affairs of the demigods. Fanfiction is a form of human creativity, and it's not something that would likely be on the gods' radar. It's more of an entertainment for the fans of the Leo Valdez stories.
Raven would probably be the first to sense that something was wrong with Beast Boy. She'd be quiet about it at first, but she'd start keeping a closer eye on him. Maybe she'd make him some herbal tea that she believes has healing properties.
His friends would be extremely worried. They might immediately drop whatever they are doing to rush to his side. For instance, Thomas would be right there, trying to assess the situation and figure out how to help. He would be filled with a sense of urgency and determination to get Minho the care he needs.
The more open - minded friends like Grover could be immediately supportive. Grover might see the love between Leo and Percy and be happy for them, maybe even helping them deal with any negative reactions from others.
Sakura, being a medic - ninja, would likely be very concerned and immediately start trying to diagnose and treat Naruto. She might be constantly by his side, using her medical skills to ease his symptoms.