(Homo sapiens necromantis)
There are a wide variety of “undead” human variants, which do not involve any sort of major changes to the genome itself, but rather rely on the use of nano-machines and/or Elementai to create a state of undeath and immortality.
Vampires (Homo sapiens necromantis sanguis)
Vampires are a peculiar human aberration. Rather than relying on the Elder genetic engineering to render them immortal, Vampires are suffused with nano-machines that can rapidly regenerate tissue and act as a superior immune system, granting Vampires a particularly effective means of immortality, and making them especially difficult to kill. However, as a cost, Vampires must be fed a steady supply of blood, due to the nano-machine infusion process destroying a subject’s bone marrow to deactivate the natural immune system. Various other bodily processes also change over time, as they naturally die off with time, while the nano-machines only perpetuate certain essential functions. Thus, Vampires suffer from long-term deficiency of melanin and whitening of hair. Over time, this can have the appearance of a patient undergoing chemotherapy.
Due to the advantages spelled out in the following articles, many vampires prefer the blood of young humans as they allow the vampire to partially reverse the aging process.
http://www.newscientist.com/article/dn23511-young-blood-reverses-heart-decline-in-old-mice.html
http://www.nytimes.com/2014/05/05/science/young-blood-may-hold-key-to-reversing-aging.html?_r=1
As the nano-machines are capable of replication, it is possible for a vampire to turn a normal human into a vampire through sharing of blood.
Zombies/Ghouls/etc. (Homo sapiens necromantis parasitis)
Unlike with Vampires who retain normal cognitive function, there are a wide variety of nano-machine-based forms of undeath that are much more destructive to the free agency of the subject. The most well known of these is the typical zombie, whose mind and body have both been taken over by the nano-machines and their “necromancer” controller.
The parasitic nature of the nano-machines of this form means that the undeath state can be transferred to any species with organic components. This “disease” of undeath thus can be spread in a manner similar to any other microorganism. Upon direct fluid transmission, the nano-machines invade the host subject, who undergoes very serious, disease-like symptoms in the process. As the nano-machines are exceedingly potent, the only “cure” is generally highly advanced medical technology (white or holy magic), or the destruction of the “necromancer” controller, who usually uses some form of Elementai (A.I. “Spirit” that interfaces with the nano-machines). In some cases however, rather than disabling the undead, the destruction of their controller, “releases” them from being bound and they become “wild” or “feral”, which can actually be quite dangerous.
In the End of Magic saga, the vast majority of undead are brought about by the use of Elementai-infused nano-machines, and so the sealing away of the Elementai is effective in bringing most of them down.