That's right people, cats. Cats responsible for demonic possession. It all fits. Think about it.....in the movie, Emily Rose's mother says that the girl was always bringing home stray cats. That was how the power of satan were able to reach her. As their numbers grew, they were able to more effectively channel their abyssal powers through the use of a black cat coven. The mother even says that they had 11 cats in their home at the time of the girl's death. That's practically inrefutable proof that they were behind her possession by Satan.
I'm sort of surprised the priest didn't sense the source of the corruption but we all often miss what is right under our noses. If only he had realized the danger they were all in he could have purged the cats in God's holy flames and saved the girl. I mean, the cats even attack and try to stop the exorcism. He should have known then their demonic natures and dealt with them accordingly.
I abjure thee, felines of darkness. Prepare to return to thy foul master in the pits of Hell.