FAQ: WHAT IS A ZOMBIED HOST?

Zombied Computer definition:
zombie computer (often shortened as zombie) is a computer attached to the Internet that has been compromised by a hacker, a computer virus, or a trojan horse. Generally, a compromised machine is only one of many in a botnet, and will be used to perform malicious tasks of one sort or another under remote direction. Most owners of zombie computers are unaware that their system is being used in this way. Because the owner tends to be unaware, these computers are metaphorically compared to zombies.

Zombies have been used extensively to send e-mail spam; as of 2005, an estimated 50–80% of all spam worldwide was sent by zombie computers. This allows spammers to avoid detection and presumably reduces their bandwidth costs, since the owners of zombies pay for their own bandwidth.
For similar reasons zombies are also used to commit click fraud against sites displaying pay per click advertising. Others can host phishing or money mule recruiting websites.