A Zombie is a malicious program that is installed on a device that transforms it into a “zombie” that attacks other systems.
A computer or other device transformed by zombie malware is first infected by a virus or Trojan. It is then deprived of its intended motivations, and instead acts in the service of the zombie (or that program's creator). It then acts as a bot or is joined into a botnet and performs mundane but harmful automated attacks. Zombie computers and assemblages of them, for example botnets, often conduct e-mail spam, silent click fraud against pay-per-click (PPC) advertising, theft of financial or personally-identifiable information (PII), and [distributed] denial-of-service (DDoS, DoS) attacks.
Specific zombie programs throughout to be destroyed, but which can still pose a threat if found on unattended devices for example, include Conficker, Flashback, Storm, Windigo, and Zeus. A novel form of zombie that arose in the early 2010s was cryptocurrency-mining malware that transformed host computers into bitcoin-mining zombies. It hijacked the computing power of laptops, then sufficient to mine digital currency. The coin was then sent to the zombie's creator who profited off of others' competing power and electricity.
“My friend’s Macbook got infected with malware and now his computer is a bitcoin-mining zombie. Does anyone know how to dislodge this from his system?”