To realistically portray stomach flu and diarrhea in fanfiction, consider the emotional aspect too. The character could be embarrassed, especially if they're around others when it first hits. There's also the frustration of being ill. They might be worried about getting better in time for an important event. And if they're in a group or with a partner, how those people react can add depth to the story. For instance, a caring friend might bring them medicine and stay to take care of them.