Headscratchers / Supernatural S01 E09 "Home"

• In this episode, Azazel broke into Mary's house, slashed her stomach, pinned her to the ceiling and set the whole house on fire. According to Missouri, she thinks that Azazel was "that kind of evil" who leaves wounds, which get infected with paranormal energy in the house. This causes a poltergeist to become attracted to it. How did the poltergeist came into the Winchester house? Why did the paranormal energy attract the poltergeist?