Anomaly Adjustment Agency
While the taxonomy is in dispute, the Undead may be described as vertebrates which have been infected with some form of disease which takes over the body and mind of the subject. After the process which creates the Undead, the creature no longer requires oxygen to function.
Types of Undead
Vampires have uniformly lost their empathy for their fellow man, but have gained great speed and strength, an impressive capacity to regenerate, and the ability to turn others into Vampires with their bite. They can be slowed by a stake through the heart, and killed by beheading.
Zombies are slow, single-minded beings who exist only to feed on living flesh. They are reportedly tough to destroy.
Other types of Undead are rumored to exist, but we have no solid information to base further instruction on.