A small rodent (gerbil) which manages to burrow beneath the skin of its human host. Upon entering the victim's epidermis it feeds, mostly on bone and ligament. Very quickly, the gerbil makes its way to the abdomen where it severs it's host's the spinal column and skoaches the spinal fluid dry. Gerbils seldom burrow into humans on instinct, usually the rodent must be exposed to some sort of radio active material in order to become a sub-dermal gerbil. A sub-dermal gerbil usually enters its victim from their ankle region, as this part of the bod is usually closest to the ground. Victims have the best odds of survival if the sub-dermal gerbil enters through one of their limbs, but only a finite amount of time to cut the gerbil out using a skiv before it reaches their abdomen.
My grandfather died from a sub-dermal gerbil attack. He promptly got out his knife, but the little rascal entered just below his flummers, burrowing deep into his gut where it was too late.