Davis/Adam/Wesley (Johnathan Rhys Meyers) doesn't have multiple personality disorder, he is actually a priest who was cursed long ago by a mountain witch, and now must feed on the souls of "non-believers" (people who are not religious) to survive.
Dr. Cara (Julianne Moore) doesn't believe in MPD and tries to solve his mental issues with rational explanations. She discovered he is actually a cursed priest and the other personalities are the souls of his victims.
Meyers infects and kills her friends, father, brother and finally her daughter. Moore kills him after he consumes her daughters soul. As she cradles her dead daughter, her daughter comes back to life. However, she is humming a tune that implies that the priest, and all of his victims souls now reside in her daughter..