Computer viruses have been around for many years. In fact, in 1949, a scientist named John von Neumann theorized that a self-replicated program was possible [source: Krebs]. The computer industry wasn't even a decade old, and already someone had figured out how to throw a monkey wrench into the figurative gears. But it took a few decades before programmers known as hackers began to build computer viruses.
While some pranksters created virus-like programs for large computer systems, it was really the introduction of the personal computer that brought computer viruses to the public's attention. A doctoral student named Fred Cohen was the first to describe self-replicating programs designed to modify computers as viruses. The name has stuck ever since.
Old-school Viruses
Some of the earliest viruses to infect personal computers included the Apple Viruses, which attacked Apple II computers
and the Brain virus, which could infect PCs.
¬In the good¬ old days (i.e., the early 1980s), viruses depended on humans to do the hard work of spreading the virus to other computers. A hacker would save the virus to disks and then distribute the disks to other people. It wasn't until modems became common that virus transmission became a real problem. Today when we think of a computer virus, we usually imagine something that transmits itself via the Internet. It might infect computers through e-mail messages or corrupted Web links. Programs like these can spread much faster than the earliest computer viruses.
We're going to take a look at 10 of the worst computer viruses to cripple a computer system. Let's start with the Melissa virus.
In the spring of 1999, a man named David L. Smith created a computer virus based on a Microsoft Word macro. He built the virus so that it could spread through e-mail messages. Smith named the virus "Melissa," saying that he named it after an exotic dancer from Florida
Daniel Hulshizer
A courtroom photo of David L. Smith, te alleged creator of the Melissa virus.Rather than shaking its moneymaker, the Melissa computer virus tempts recipients into opening a document with an e-mail message like "Here is that document you asked for, don't show it to anybody else." Once activated, the virus replicates itself and sends itself out to the top 50 people in the recipient's e-mail address book.
The virus spread rapidly after Smith unleashed it on the world. The United States federal government became very interested in Smith's work -- according to statements made by FBI officials to Congress, the Melissa virus "wreaked havoc on government and private sector networks" [source: FBI]. The increase in e-mail traffic forced some companies to discontinue e-mail programs until the virus was contained.
After a lengthy trial process, Smith lost his case and received a 20-month jail sentence. The court also fined Smith $5,000 and forbade him from accessing computer networks without court authorization [source: BBC]. Ultimately, the Melissa virus didn't cripple the Internet, but it was one of the first computer viruses to get the public's attention.
Flavors of Viruses
In this article, we'll look at several different kinds of computer viruses. Here's a quick guide to what we'll see:
• The general term computer virus usually covers programs that modify how a computer works (including damaging the computer) and can self-replicate. A true computer virus requires a host program to run properly -- Melissa used a Word document.
• A worm, on the other hand, doesn't require a host program. It's an application that can replicate itself and send itself through computer networks.
• Trojan horses are programs that claim to do one thing but really do another. Some might damage a victim's hard drive. Others can create a backdoor, allowing a remote user to access the victim's computer system.
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