Nanobots, created either by renegade scientists or a race of reptilians, were introduced into a handful of human bodies in order to repair damaged cells. These nanobots performed so well that they rendered their hosts immortal.

However, the   Nanobots themselves are not immortal and must self-replicate by utilizing the iron atoms from the hemoglobin in the host's red blood cells.  The result of this nanoreplication process is the constant need for sufficient supplies of blood.

Unable to keep up with the demand, the host has no choice but to seek out blood from others.  If the colony of Nanobots exceeds the host's ability to supply sufficient RBCs, some nanobots may migrate into another host, usually the next victim of the primary host's bite.