Zombies (often mislabeled as zomboys and zombees) are a form of being in MarrissaTheWriter stories. They are not a separate species, as a human can turn into a zombie by the use of zombee taters.
Typically, when a human consumes zombee taters and dies, they then turn into a zombie. This so far has happened only to Chell and various Andord Hell residents. In addition, a zombie that bites another person turns them into a zombie, and Atlas is turned into a zombie in this fashion. Zombies can be converted back to normal by transporting them to Andord Hell.
The humans with speshul powers are exempt to this rule, as when they consume zombee taters after their death, they just become alive again.
Zombies first appear in ITS MY LIFE!, as Chell becomes one via the zombee taters (as Marrissa Roberts learns with her super-detective power). She then bites Atlas, turning him into a zomboy. Marrissa then throws Chell and Atlas into a portal to Andord Hell, where they become normal, seeing as "there are no zombees in Andord Hell". Nevertheless, Atlas rejoins his robot mate P-Body and they begin throwing zombee taters into Andord Hell, turning its residents into zombies, as well as creating The Ultimate Zombee.
The zombee taters come into play once again, as Chell has a relapse of brain damage and GLaDOS reveals that the only way to cure it is via the taters. Marrissa thus has to choose between a zombified Chell and a brain-damaged Chell. Later, this point is negated as Chell and GLaDOS fuse into CHELLGADOS, which in turn cures Chell's brain damage.
Still later, when Marrissa is shot by Atlas and P-Body, Oracle Torrent reveals to Wheatly the speshul power mentioned above: if Marrissa were to eat the zombee taters, she would become alive again. Thus, Wheatly and Oracle Torrent begin stuffing Marrissa with the zombee taters and thus resurrect her.
The zombee taters also briefly come into play in the ASBusinessMagnet comic Sweet Chell and Hella Marrissa. There, the zombee taters are used in a similar manner to ITS MY LIFE!, except this time they come from the Moon and not Portal Labs. However, in an alternate version of the comic in question, the zombee taters turn out to be too big and thus blow up the Earth, creating an alternate timeline.