Wait... did I miss something (SPOILERS)
Eric told David that to purify and free Mia of her possession and ending the curse is to bury her alive, burn her alive or dismember her.
David buried Mia and brought her back to life and she was back to normal.
I was under the understanding this would end the curse and send it back to where it came from.
Yet Eric who died under the house shows up suddenly, possessed like Mia was earlier, and stabs David.
When did Eric get for the lack of a better word, infected?
Was it when Mia stabbed him with the box cutter when Eric knocked possessed Mia unconscious and saved David from drowning?