Computer security experts are in a race to find ways to block increasingly complex worms and viruses that burrow into computers, usually via e-mail, and wreak havoc on network systems.

A computer worm is a reproductive program that can travel on its own through network connections. A virus, on the other hand, depends on the transfer of files between machines to spread. The damage they inflict can range from headaches for computer users if their programs are disabled, to billions of dollars' worth of losses if businesses are unable to function normally.

Computer worms and viruses are as old as the Internet. The Morris worm of 1988 is the first known malicious worm. It was released into the ARPAnet -- the federal Advanced Research Project Agency's precursor to the Internet. The worm copied itself across the ARPAnet overnight, crashing 6,000 machines. A graduate student reportedly created the worm out of boredom.

The Melissa virus struck in 1999, entering computers via a Microsoft Word document attachment on e-mails with the seemingly innocuous subject line, "Important message from [name]," and causing millions of dollars in damages.

In 2002, the vicious Bugbear worm entered the scene. Once activated, Bugbear checked for antivirus software or personal "firewalls" and then worked to destroy them. According to Network World Fusion, the worm then implanted a "Trojan," which created a listening service so attackers could upload or download files at will.

"Trojan horses" are usually sent via e-mail, often disguised as programs that claim to provide online services, and once activated can read information from inside computer files.

The SQL Slammer worm attacked networks across Asia, Europe and the Americas, closing certain ATM machines for half a day in January 2003 and becoming the focus of an FBI investigation, CNN reported. Continental Airlines had to resort to booking flights with pen and paper when the worm temporarily disabled its computer systems.

A variant of the Blaster worm, which hit more than 1 million computer systems worldwide in August 2003, shut down the mail-tracking systems of Japan's post offices for hours in October, according to News.com.au. The infected computers were immediately disconnected from the network to avoid more widespread damage.

Computer viruses and worms cause huge economic losses each year. Based on surveys, PricewaterhouseCoopers estimated in 2000 that global corporations sustained more than $1.5 trillion in losses in one year due to security breaches, including the culprit, computer viruses. Sixty-five percent of respondents said they experienced loss of productivity due to the cyber-attacks.

The widespread use of the Windows operating system and Microsoft operating tools has made them a target of computer viruses. Microsoft provides "patches" that computer users may download onto their machines to battle specific viruses. The issue is so significant that company founder Bill Gates is working on an initiative aimed at improving the security of Microsoft products.

The problem has become so profound that Microsoft announced Nov. 5, 2003, a new $5 million reward program to help law enforcement catch and convict people who release worms, viruses and other types of destructive computer programs. But often worms and viruses spread too quickly for investigators to find their creators.

The common environment created when certain computer programs become dominant has made it easier for computer users to communicate with each other around the world but also has created a fertile ground for computer viruses. Creating a more diverse software environment would make it harder for viruses to spread but many say is not worth the toll it would take on productivity and efficiency.

-- By Larisa Epatko, Online NewsHour