Outside of horror stories populated by brain-munching walking-dead freaks, the term "Zombies" refers to a computer infected with a spambot and controlled as part of a larger botnet by "bot herders". Computers infected by spambots may show no obvious symptoms of infection as anything to alert the user could result in the removal of the software.
The term Zombie is apparopriate in that the tasks carried out by Zombie machines would be unknown to the user.
