Europe Has the Most Zombies
EmailBattles.com has an article about how Europe has the most zombie computers.
Zombie computers are computers that have been taken over by malicious software. Zombies are used for various unwholesome purposes, such as the creation of botnets — massive networks of zombie computers that combine computing power and bandwidth to distribute spyware, send spam, and perform other bad activities. One botnet was found to contain 1.5 million zombie computers. It was controlled by only three persons, and as the article said, "...it was just a drop in the ocean". That gives some idea of the magnitude of the problem.
Computers can be turned into zombies through trojan horses. They are just another example of the high security risk of using public computers that have thousands of different people using them. You just don't know what kinds of bad things the computer might be doing. Fortunately you can entirely bypass the infected state of a public computer by using a Linux live CD.
Zombie Computer Statistics
If you are interested in seeing live statistics about zombie computers, check out CipherTrust.com.