Just to add in another thought to my earlier post, I assumed you already knew not to use the toothbrush, but rather you were concerned if you had used it before your roommate told you what was done, then you wanted to know if you would get ill. But just in case I was mistake about your question, I agree with everyone else, do not use that toothbrush, of course! Throw it away, buy another and secret it away from the roommate. Some roommates are nice people and some are not. If you can throw them out or move away yourself, then do so. Life is too short to waste it on maniacs like that.
buy a new toothbrush and suggest that if your room mate does not grow up - he or she may be looking for alternative accomadation.
Many things can happen to you if you use this toothbrush. Bacterial infection, exposure to parasites and worms and god knows what else lives on the rim of a toilet. Buy a new toothbrush and consider getting a new flatmate.
GET A NEW TOOTHBRUSH! There can be so much bacteria under the rim of a toilet, that it is not worth you getting ill. Then get a new roommate!!! Take what she says seriously, and don't risk your health on this nut case!
I hope you don't have that roommate anymore. Gee whiz, what a nutcase. You probably won't get sick from it, mainly because the amount of bacteria you may have taken in would have been small. But if you find yourself feeling really sick over the next few weeks, then of course go to the doctor and they can do some blood tests and see if you have an infection from that toothbruth, and they can treat you for it. As for what your roommate said about the dog's behind, that was obviously a lie.
Just go to the dollar store and buy a new toothbrush...you never know what could be in the toilet rim or on a dog's behind (worms most likely would be the worst)...for now put toothpaste on your finger and use it like a tooth brush until you can go to the store...I'm sorry your room mate did that...that's disgusting.