If you sided with the vampires in the Dawnguard DLC, but don't kill Isran, the Dawnguard send vampire hunters after you periodically, even if you cure your vampirism. This just edits the conditions for the vampire hunters random event to include the player currently being a vampire. The Dawnguard will still be hostile if you go visit them, they just won't send hit squads after you. 

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