Ratting out
It was a long day for Roy, he had just come home to his apartment after quite the long day at work and there was nothing he looked forward to more than just settling down on the couch and watch some tv just to relax.
He figured maybe he'd be more relaxed if he was in just his underwear and socks. "It's not like I'm having any company over and my roommate's gonna be away for some time..."
So he took off his shirt and pants leaving him barely naked, he put his dirty clothes in the hamper and headed into the kitchen.
"Gotta be some snacks in here" He thought. 
Opening up the refrigerator, he catches sight of something yellow.
It was a large chunk of cheese, he picked it up while food shopping last week, the store told him that it was some kind of brand new product called squeaky cheese but they hadn't figured out what company made it or the name of the company.
However he had been compelled to buy it after noticing the front package depicting a cartoon rat winking with a happy face and figured maybe he ought to get a few packages of it.
He picked up the cheese, grabbed a bowl of chips and some soda and went into the living room with his snacks.
As he turned on the TV, Roy catches sight of an old classic, Tom and Jerry.
He sits down on the couch to relax but as he eats the chips, he keeps noticing how powerful the scent of the cheese he took out is.
Grabbing for the cheese, Roy sniffs at it a bit before taking a big bite into it.
Munching away, he is so enamoured by the taste of the squeaky cheese that he didn't even realize that his ears twitched and grew bigger, they were the size of classic record discs.
Roy began feeling an itching around his cheese smeared lips but shrugged it off as he was enjoying his snack too much.
He ate the cheese again and then noticed that a large, bulbous growth appeared close up to his sight. If he was looking in a mirror, he'd have seen that his nose had grown large, pink and button like.
A voice in his head had begun yelling at him to stop eating the cheese, but the scent was overpowering and he just ignored the voice and ate some more.
Soon the gray fur that had grown all over his face began spreading down his body, it was now all over his chest and back but it then went down all over his big belly which seemed to be getting bigger
Another bite with his newly formed buck rodent teeth and the fur spread onto his arms and then his hands became a more pinkish color.
"Mmm... real good cheese that's for sure." Roy chuckled as he licked his lips getting some of the cheese.
That was when his tail started to grow, ripping out of his underwear, it was very long and was flicking back and forth as Roy was continuing to enjoy his show and eat his snack.
Soon the fur was spreading down to his legs while underneath his socks his feet were turning the same color as his hands.
Roy then laughed as the mouse on TV played a cruel prank on the cat while his blonde hair started to turn the same color as his fur.
"Man! That was so funny! *Squeak!*" Roy laughed with a bit of a rodent squeak,  he really adored that mouse and his cunning tricks.
Just then he felt rather uncomfortable as he looked down and all around his large rodent body.
He then realized what was bothering him and he took of his underwear which was pretty much ruined by his tail and tossed them aside
"*Squeak!* Oh yeah, that cheese sure was the best!"
He headed over to the fridge, opened it up and looked at all the packages of squeaky cheese he had.
"Hmm... I doubt I'll ever be able to eat all this cheese by myself *Squeak!* Maybe my roommate will want some..."
Awhile later, someone else walked into the apartment, Roy's roommate, Johnny.
"Hey Roy, how was... Huh!?"
Johnny got the shock of his life when he caught sight of a large anthropomorphic rat sitting on the couch in the same spot where he usually saw his roommate.
"Hey, Johnny. How was work?"
Rubbing his eyes, Johnny blinked a few times at the fat rat. He then realized that it was talking in the same voice as his roommate!
"Roy!? Is that you!?"
"Yup! Me in my huge ratty glory *Squeak!*"
The poor guy was in shock over what happened to his friend. "Oh my god, I don't know what is going on but you must be really sick! I should call the doct-"
He paused for a moment as he caught a whiff of cheese "Oh wow, that smells like some pretty good cheese."
"That's actually me you're smelling *Squeak!* But there is plenty of cheese to go around if you wanna sit on down next to me."
Johnny absent mindedly nodded as he walked over to the large rat and sat next to him on the couch.
"Thatta boy! Here ya go!" Laughed Roy with a few squeaks as Johnny took some cheese and began to eat.
And as his roommate's ears, teeth and tail took form, Roy smiled and found himself wanting to thank the people who made squeaky cheese for coming up with such a fun new product.