ZOMBIE
A zombie computer is a machine infected with malicious code intended to give remote control to an attacker. The latter attempts to infect as many computers as possible in this way and to control them all within a single network called a "botnet". Each member of this network is metaphorically called a zombie because it is no longer under the real control of its owner, although the latter generally does not notice anything. A botnet is a valuable asset for an attacker, because it allows him to generate income by renting control of all his zombie computers to carry out denial of service attacks (connecting en masse to a website in order to make it inaccessible), to install malicious code that will allow the theft of identifiers and banking information, or to host phishing sites.
It is impossible to visually identify a zombie computer. Stale food residue on the keyboard and dried coffee stains are neither evidence of infection nor a good reason to change it.