Introduction

No doubt, you have heard of a widespread virus/worm epidemic. The past few years have left us with many headliners: The Melissa, I Love You, Code Red, and Nimda worms have reportedly caused millions of dollars in damage. Other notables include Anna Kournikova, Magistr, Goner, BadTrans, and Kak, among others. New variants creep up every day. The anti-virus industry has grown to be extensive and profitable. But what exactly are they deriving their profit from? The answer: the propagation of malicious code.

Of course, the anti-virus industry has expanded beyond just viruses they now catalogue and analyze Trojan horse programs (or Trojans for short), worms, and macro "viruses."

How Do Viruses, Trojans Horses, and Worms Differ?

Malicious code (sometimes referred to as malware, which is short for "malicious software") is usually classified by the type of propagation (spreading) mechanism it employs, with a few exceptions in regard to the particular platforms and mechanisms it requires to run (such as macro viruses, which require a host program to interpret them). Also take note that even though the term malicious code is used, a virus/Trojan/worm may not actually cause damage. In this context, malicious indicates the potential to do damage, rather than actually causing malice. Some people consider the fact that a foreign piece of code on their systems that is consuming resources, no matter how small an amount, is a malicious act in itself.