"My friend came up with a theory that there is a universe parallel to this one. The people there are the same and and do the same things as us; its a reflection of this world.
Complex part...
Mirrors are the portals. Your opposite in that world unknowingly blocks you from entering. For example: You reach out and touch the mirror and so does your opposite, each blocking the other from passing through.
Every now and then a paradox occurs: A person dies, but their opposite doesn't (usually because of preternatural intervention) they are able to pass through. They kill somebody on the other side, but the opposite on their side DOESNT DIE.(No opposite doing exactly what they do) They are now a paradox as well.
They don't have a reflection anymore, they make the same thing happen to other people (make it spread so to speak)... you guessed it
Vampires."