Read up on how computers can become infected by stealthy intruders, in this instance RATS, or remote access Trojans, courtesy of Paul Ducklin on the Naked Security blog.
Zombie is another name for RAT. “Indeed, the name RAT was originally coined as a metaphor that referred as much to the criminals that deployed the malware as to the malware itself.
“But few RATs were ever just about surreptitious access to webcams and screenshots.
“Remote access tools of this sort are more generally known as bots, short for software robots, or zombies, because they lie in wait for commands to arise and wreak havoc.
“And almost every zombie out there supports, in addition to any built-in features such as file stealing, screen capturing and webcam snooping, a generic command by which it can update and replace itself with completely new malware, or download and install new malware to run alongside itself.”
A link to the full SophosLabs report on RATS is provided. It’s well worth digging into.