Zombies are a species residing in the Human Realm that are seemingly restricted to certain areas, such as the Z-Zone that John mentions. It seems that the Vatican takes responsibility for exterminating the species, likely because they are deemed blasphemous by the church.
Physiology and Appearance
Zombies are likely humans that have caught the virus or been infected somehow to become zombies. As such, they have the same physiological characteristics as humans except they appear to be decaying and are of various, unnatural skin shades.
They appear to be emotionless. They seem to flock towards humans for consumption with no purpose otherwise.
They seem to have little mobility and no reaction to attacks.