Basic Virus Information:

The first viruses emerged in the mid-1980's. By 1990, there were still less than 100 viruses. Today it is estimated that there may be more than 50,000 viruses. Interestingly, the majority of viruses are not out in the public, referred to as "in the wild." Resources say that only 100-180 of the 50,000 viruses account for all the viruses that are in the wild. Most of the viruses exist only in personal virus collections, also called zoos.

A computer virus is simply a computer program. It is intentionally written so that it will spread by creating copies of itself and attaching the copies to other files. Any program that does this is considered a virus. Viruses generally reproduce without the user's knowledge or permission. Viruses are written with the intent of damaging other computers or annoying other users. They can be written by anyone with programming knowledge.

As the name implies, a Trojan Horse program comes with a hidden surprise intended by the programmer but totally unexpected by the user. Trojan Horses are often designed to cause damage or do something malicious to a system, but are disguised as something useful. Unlike viruses, Trojan Horses don't make copies of themselves. Like viruses, they can cause significant damage to a computer.

Worms are like viruses in that they do replicate themselves. However, instead of spreading from file to file, they spread from computer to computer, infecting an entire system.

Worms are insidious because they rely less (or not at all) upon human behavior in order to spread themselves from one computer to others. The computer worm is a program that is designed to copy itself from one computer to another, leveraging some network medium: e-mail, TCP/IP, etc. The worm is more interested in infecting as many machines as possible on the network, and less interested in spreading many copies of itself on a single computer (like a computer virus). The prototypical worm infects (or causes its code to run on) a target system only once; after the initial infection, the worm attempts to spread to other machines on the network.