Two of the most insidious dangers on the Internetzombies and Trojan horsesare so good at disguising themselves that you may never know they've infected your computer. Yet millions of computers are unknowingly infected with these pieces of malware.
Zombies and Trojan horses (generally just called Trojans) are related dangers, and some people lump them together. But, in fact, they are usually used for very different purposes.
A zombie, sometimes called a bot (short for robot), is a computer that has been taken over by someone else to do their bidding. The reason for the name is obviousin noncomputer terms, a zombie is someone who has been dead and then resurrected to do the bidding of a zombie master. The zombie has to follow the commands of his master and has no will of his own.
A computer zombie operates in much the same way. First it is infected, often by opening an infected piece of email or perhaps by visiting a website that plants the infection on the PC. The PC typically shows no outward signs of infection. Most of the time, the computer acts normally. But a zombie master can issue commands, telling it to take actions. Usually, that action involves something illegal, such as sending out spam or phishing attacks or planting spyware on another PC.
A zombie on its own cannot do much damage. But zombies do not act on their own. Instead, they act in concert. People command huge networks of zombies that number in the tens of thousandssometimes in the millions. Those who own the networks rent them out to send massive amounts of spam or phishing attacks, and these people are paid well for their work. Most of the time, the people whose PCs are infected do not even know their PCs are zombies and are sending out spam and phishing attacks.
A Trojan is slightly different from a zombie. A PC can be infected by a Trojan in the same way that a zombie can infect a PCby email, by visiting an infected website, and so on. Often, a person might think he is downloading a useful piece of software, but in fact, a Trojan lurks within. This is where the Trojan gets its name, from the mythological Trojan Horse that appeared to be a gift to the people of Troy but in fact contained the Greek army, crouching in its belly and waiting until nightfall to come out and sack the city.
A PC infected by a Trojan often doesn't show signs of infection. But it allows someone to take over complete control of the PC, just as if she were sitting at the keyboard. So, someone can steal information and files and use the Trojan to launch attacks on other PCs and websites. Trojans can also automatically download spyware to the infected PC or steal personal information and send it in an email to a hacker.