What is a Virus, Worm, and Trojan Horse?

Posted By BrokenClaw on February 2, 2008

Computer viruses, worms, and trojan horses are all malicious computer programs which get copied onto your computer without your knowledge. There are certain characteristics which differentiate them from each other, but they are all bad for you as a user and for the Internet as a whole.

A computer virus is software that gets actively transferred onto your computer as a piggyback on another program — kind of like a biological virus piggybacks itself on a respiratory or blood droplet. In earlier times, the main method of infection was from floppy discs that were shared between computers. As networks, including the World Wide Web, became ubiquitous, file transfers across the network made virus transfer even easier. Once the virus program infects a machine, it can have a wide range of effects.

The most benign viruses did nothing more than print something on the screen like, “Haha, you’re infected!”, or they played a sound or video to announce themselves. The days of the happy virus are long gone. Today’s viruses are more malicious. They can delete files from your computer and make your computer completely unusable. All computer viruses are self-replicating, just like their biological namesake. They make copies of themselves so that they can be transferred to other computers.

A computer worm is similar to a virus, but it is even more dangerous, because its defining characteristic is that it can crawl from computer to computer without you doing anything — like a worm crawling through the soil. The concept of a computer worm was actually developed as a diagnostic tool for network administrators. Unfortunately, the same type of software became a tool to transfer malicious software automatically around any network, including the Web.

A trojan horse, sometimes referred to simply as a trojan, is a type of malicious software that attacks your computer in a certain way. Its defining characteristic is that it appears to be some other type of program that can stay dormant on a machine for a long time until some event triggers it to act. Its name, of course, comes from the story of ancient Troy, when the gift horse statue turned out to be a clandestine attack.

A trojan horse can be either a virus or a worm, but the distinction is really semantic, and the result is the same. They are often transferred as files attached to email, purporting to be some enticing goody, but which are actually an executable programs. A typical example is a screen saver, a file with an extension of .scr which doesn’t run until you activate it on your system.

Read what happens with a virus on your computer or viruses on the Internet.

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