Shiki

Shiki (屍鬼) are supernatural, reanimated corpses inhabited by deceased spirits. They feed and survive on the blood of humans. Shiki are corpses that have risen up after dying from having their blood drained by another shiki.

Fan-made
In Mega Man ZX Shippuden, The shiki came into existence when Yahweh Ōtsutsuki, Yula Ōtsutsuki and Suzugamori Ōtsutsuki have to reanimate the corpses that were found in the remains of the Great Grand Civil War. They gave the reanimated corpses the ability to survive by draining the victim's blood as special magic that grand them immortality.

Humans with extraordinary or powerful personality traits can transform into jinrou, a superior subspecies of shiki. Jinrou are almost indistinguishable from ordinary humans.

Anime
It is unknown how shiki came into existence, though they can turn humans into shiki by draining the victim's blood. The biggest possibility is that the Shiki simply came into existance by magic that makes more of them, one Shiki had to start it somehow. It is presumed that the bite of a shiki infects humans with a venom that slowly changes the victim with each subsequent bite. Once the human has been bitten several times, they eventually die through hemorrhaging. Toshio Ozaki saw that bitten patients display anemia, extreme fatigue, and drowsiness. During Kyouko's transition, Toshio observed that the process of dying and rising up as a shiki takes around one week.

Not all bitten humans rise up as shiki. None of Nao's family reanimated, despite having their blood drained. This indicates that the bitten human must have a quality about them that corresponds to reanimation. It seems that overall passion towards life is a key factor in rising up and sometimes even unfinished bussines, as Megumi Shimizu, fed up with village life and obsessed with city life, rose again as a shiki.

Humans with extraordinary or powerful personality traits can transform into jinrou, a superior subspecies of shiki. Jinrou are almost indistinguishable from ordinary humans.