Stuff to know before reading:
This is related to an AU I have where Bill was transformed into a 12-year-old due to a magical accident and got stuck that way. Since he had nowhere else to go, Stan begrudgingly let him stay at the Mystery Shack under the pretense that he didn't cause any deaths (directly or indirectly). He usually sleeps where ever he happens to pass out late at night/early in the morning (which has been one of the twin's beds a few times), because he's not used to needing to go to sleep and often forgets to until his body forces him to.
Summary (Tumblr Version):
No one knew he could,
But he still did: Bill got sick.
Karma is a bitch.
It had been a while since Bill had gotten stuck in the body of a preteen child, and while it was usually fun for him to get into trouble any way possible and be a general nuisance, today he had been unusually quiet.
That is, for the short while he had been around.
The oddity started at breakfast. Usually Bill would have been woken up by the morning ruckus, and soon appear in the kitchen while putting in his two cents about whatever happened to be going on, but there had been no sign of him. To make sure that he was actually still somewhere in the Shack, the twins set out on a hunt for him. It wasn't long before they found Bill sleeping on the floor in front of the sofa in Soos' break room, and Mabel gave him a rather rude awakening.
After the demon had quietly eaten breakfast, he practically disappeared. With him missing, Stan and Dipper started getting anxious, worrying that Bill might be planning something, though the others reassured them that is was probably nothing.
Wendy's guess was that he was simply trying to catch up on sleep, so he was hiding somewhere he wouldn't be disturbed.
It wasn't until late afternoon when they heard some very loud moaning that they discovered the real reason.
Running upstairs towards the source of the noise, the twins found Bill on Dipper's bed curled up in the fetal position. With his red cheeks, wet eyes, and runny nose it looked like he'd been crying.
"Uh… hey Bill, everything okay?" Dipper asked cautiously, keeping some distance between them in case it was a trap of some sort.
Bill groaned pathetically, then snapped at him, "Nothing is okay, Pine Tree! I'm gonna die in this melting fleshy prison!"
Dipper let out an annoyed sigh. Great. Bill was having a mortality crisis.
"Haha, come on, Bill, you're not melting!" Mabel laughed, thinking Bill was just being a silly drama queen. She plodded over to him and reached out to pat him on the head, "You're completely fine! Boop-!" she gasped as her had touched his head.
"There's nothing fine about this! This body's head is going to explode! Be it through the top of the forehead or out the front of this face taking the nose with it!" Bill complained, coughing and he swatting her hand away to relieve himself of the additional pain the touch caused. Usually he found pain to be pretty entertaining, but that's when it was coming from outside his body, not inside.
"Dipper, come here!" Mabel gestured her brother over, then grabbed his hand and pressed it to Bill's forehead. The demon whined in protest, not enjoying the return of the pain.
Dipper nearly yanked his hand away in surprise at how hot his skin felt, "oh my god. Bill, you're sick!" No wonder his face looked like a miserable mess.
"What…?" Bill finally opened his eyes and looked at him in exhausted confusion.
"You're sick! You must have a cold!" Dipper almost started laughing, but managed to contain himself. There was something about Bill suffering like this that was really amusing.
"A cold?! No way! This feels too awful for it to possibly be a mere mortal's cold!" Bill groaned out, rubbing his head to try and relieve some of the pressure, "my head feels like it's going to split opeeeeeeen! Don't just stand there! One of you do something!"
"Heheh, okay!" Mabel giggled, also highly amused by Bill's predicament. She ran over to the door and yelled down the stairs, "GRUNKLE STAAAAAAN! BILL'S SIIIIIIIICK! AND HE'S BEING REALLY DRAMATIC ABOUT IIIIIIIIIT!"
"I AM NOT BEING DRAMATIC!" Bill yelled over her, glaring at Dipper when the boy finally started snickering.
"Sorry, you really are though," Dipper told him, covering his mouth with his hands to muffle the laughter.
"You're totally being dramatic!" Mabel agreed, running back over and putting her hands on the edge of the bed as she leaned over it.
Bill was about to yell at them again when Stan burst in through the door, "alright ya little brat! Come 'ere!"
The twins moved out of the way as Stan grabbed Bill and roughly pulled him into a sitting position, earning himself a lot of cursing in return, though the real fight began when he shoved the thermometer in his mouth.
"Wah ith thith, geh ih ou' of my mouf!" Bill tried to turn his head away, not happy with a foreign object suddenly being stuck inside the mouth he wasn't used to having. It didn't work very well though, as the old man was surprisingly strong and he was currently a weakened child.
"Sit still! It's just to check your temperature," Stan told him, ignoring the demon's protesting. After a moment the thermometer beeped and he pulled it out of Bill's mouth. "101.1, nothing too bad."
He had to be joking. "Nothing too bad?! This skull feels like it's being pierced by spears and I'm leaking slimy fluids out the face and you're telling me it's not tha-… tha-…. ACHOO! -that bad…"
"Hey! Cover your mouth when you do that!" Stan yelled at him, disgusted that he was now covered in snot and germs. "And get some tissues for your face, it's gross. You only have a cold so just eat some soup and take some pain killers and you'll be fine in a couple days," he grumbled as he got up and headed out the door for the bathroom, then yelled back, "And kids! Take care of him and keep him up there so he doesn't spread it around the Shack! No use getting everyone else sick!"
"Don't worry Bill, we'll take good care of you!" Mabel exclaimed, then spun around and headed for the door, "I'll go make you some of Mabel's special cure all soup! That'll fix you right up!"
Bill watched as she left, commenting to the remaining sibling, "I reiterate: I am going to die."
