The term 'Zombies' has always meant something supernatural in the past. It referred to a phenomenon known as 'the walking dead'. Gruesome! Zombies are generally regarded as not real, although there seems to be some evidence that zombies do exist, but they aren't really dead. They are under the control of a 'master' somewhere.
But today another form of zombie exists and unfortunately, this zombie is all too real. This zombie is part of a 'botnet' and is one of the main reasons our email in boxes fill up so quickly with spam and phish emails.
It all starts when an unsuspecting, uneducated or careless computer user clicks on a link in an email, generally from someone they don't know. The email may indicate it is for a greeting card or some other online goody. It may say 'Thank you for joining' some enticing sounding mail list. The user knows he or she didn't sign up for anything so wants to see what that is about. When the user clicks the link the user's browser goes to an infected (on purpose!) website and a nasty bit of programming is installed on the user's computer. Now the originator of the malware has virtual control of the computer and it is part of an evil 'botnet', a network of zombie computers.
The 'botnets' are leased to evildoers that now have control to send out literally millions of more nasty phishing emails that are designed to take control of even more computers. The ultimate goal of the botnet controllers are to steal identities, steal financial information, increase the spam list base or who knows what other evil actions.
The way to prevent your computer from joining this zombie network is to keep Windows updated, install and keep updated a anti-virus scanner, a anti-spyware scanners and delete unopened emails from people you don't know!
Mommy, how did Daddy become a zombie? An easy to understand explanation about zombies and botnets.
From PC World, Zombie PCs: Silent, Growing Threat