Vampirism. Not a virus, not a monster, but a parasite.
The parasite starts its life cycle… somewhere. The soil, perhaps. Perhaps the act of burying and unburying your dead exposes you to the dormant parasite, and you become its first host. The parasite worms its way into your brain and starts making adjustments to your physiology and behavior. Maybe it exudes its own chemical pheromones that have been adapted to be wildly attractive to humans, or maybe it boosts your own magnetic characteristics. Maybe the parasite is photosensitive, or cannot tolerate temperatures much warmer than the average human body – you find yourself avoiding the light and sticking to dark, cool places during the day. Maybe the parasite feeds upon some minerals or proteins or other components in fresh blood, so you find yourself craving human flesh to replace the nutrients being robbed from your own system. Maybe the parasite must be transferred from its primary host to a secondary host before it can reach the final stage in its life cycle. Perhaps it enters your saliva and is transferred to the bloodstream of your prey, where it consumes them from the inside and reproduces in their dying body before being returned to the soil during to start the cycle anew. Perhaps complicated burial practices aim to halt the parasite’s life cycle at this stage, and vampirism only spreads when these practices are shirked.