BOR Parasites (Biological Organ Regenerative Parasites) are engineered parasites and the source of the Revenants. They are implanted into the heart of a human corpse and eventually revive them as a Revenant. How long it takes for the human corpse to revive as a Revenant differs between individuals, it can even take years in some cases. How the BOR Parasite revives its host is by latching itself to the heart of its host and it begins sending its own blood into the host. This somehow enables the Parasite to regulate and control all of its host's organs, including the brain, from the heart. By maintaining the BOR Parasite's invasion at a certain level the host is able to maintain a perfect sense of self while still allowing the Parasite to perform all of its vital functions.
Thanks to the BOR Parasite implanted in the Revenant's heart life and death change dramatically for the host Revenant. Should a Revenant die their corpse will not rot, it will instead burst into a mist of golden particles called Haze. This is called dispersal. After a Revenant has dispersed they will regenerate over time in perfect physical health. however it is not uncommon for Revenants to suffer memory loss after regenerating from a dispersal. The only way to truly kill a Revenant is to pierce one's heart where the BOR Parasite is housed. Should this happen the cellular bonds throughout the host Revenant will dissolve and they will turn to ash.
Because of the BOR Parasite Revenants do not need to eat food or drink water to survive, however their bodies are still able to process regular food and drink as shown during Yakumo and Emily Su's reunion. Revenants only need blood, or the blood-like substance contained within Blood Beads, to survive. Should a Revenant go too long without blood or Blood Beads the BOR Parasite will stimulate the Revenant's nervous system as a reaction to the starvation of blood. This is highly unpleasant for the host Revenant and is the first early warning sign of Frenzy.
Should the host Revenant continue to go without feeding the BOR Parasite will burrow deeper into the host's heart and completely take over the host Revenant reducing the Revenant to no more than a mindless, instinct driven beast. Revenants who have suffered this fate have been named The Lost. When the BOR Parasite takes over this way true death is impossible, as is reversing the condition. The BOR Parasite will regenerate the body it took over endlessly and without fail regardless of whether or not the heart was destroyed. The cost of this for the BOR Parasite is that each time it regenerates the body it has completely taken over said body looks less and less like the Revenant it once was and more like a grotesque monster.