The worst computer virus is the one that happens to infect your own personal computer. Unfortunately, millions of us have had this misfortune, and then spent hours — sometimes days — cleaning, restoring and recovering from a computer virus.
Today, many pieces of malware are designed to attack specific targets, such as the Stuxnet worm (designed to knock out Iran's nuclear centrifuges) or the Zeus and SpyEye Trojans (designed to steal from victims' bank accounts).
But back in the day — say, before 2005 — viruses were all about how much damage could be done by trashing hard drives, corrupting files, bringing corporations to their knees and slowing down the Internet itself.
It was an atmosphere that turned computer security into a big business and created some of the most notorious viruses of all time.
Here are the dirtiest of the bunch, our terrible 10 worst viruses in history.
Before there was the World Wide Web, the first computer viruses spread via floppy disks. One of the earliest was the 1987 boot-sector virus Stoned, which taunted infected users with the on-screen message, "Your computer is now stoned."
Several variants of the virus were written by copycats, ushering in the practice of hackers updating existing virus code to create more infections.
Toward the end of 1987, the Jerusalem virus began spreading. The virus was much more destructive than the Stoned virus, infecting both .exe and .com files (different kinds of applications).
Because it launched only on every Friday the 13th, Jerusalem's spread was more slow-moving than Stoned's, but Jerusalem destroyed tens of thousands of users' programs along the way.
The Morris Worm
November 1988 saw what is widely regarded as the first worm — a self-contained program that spreads without human intervention — to infect public networks. At the time, it was estimated to have infected about 10 percent of all computers connected to the nascent Internet.
Its creator, Cornell University graduate student Robert Tappan Morris, whose father was a famous computer scientist, became the first person convicted under the Computer Fraud and Abuse Act.
The Concept Virus
The '90s saw the development of a raft of new bugs, including so-called polymorphic viruses that could change their appearance with each new infection, making it difficult for anti-virus software to detect their presence.
In 1995, the Concept virus broke new ground by being the first to infect Microsoft Word documents. Unaware users sharing documents via email helped make it one of the fastest spreading viruses of its time.
Before the decade was out, one of the worst viruses of all-time appeared. Reputed to be named for a Florida stripper, Melissa showed up in mid-1999 and was one of the first viruses designed to spread from computer to computer without relying on action on the user's part.
For every PC it infected via email, it attempted to infect another 50 using the victim's Microsoft Outlook address book. The subsequent volume of Internet traffic forced companies such as Intel and Microsoft to temporarily shut down their own mail servers.
The Love Bug
Social engineering — tricking a person to open a file or reveal information — came into its own in May 2000 with the ILOVEYOU virus.
Like Melissa, it also used email and appeared to come from someone known to the recipient. But in reality, the attached script deleted multimedia and personal files, changed the Internet Explorer start page and unleashed a torrent of junk mail.
The Love Bug is still considered to be one of the most destructive viruses ever. It infected more than 50 million computers in just nine days, and caused several military sites to shut down their networks until the virus could be purged.
The Anna Kournikova Virus
You didn't have to be a tennis fan in February 2001 to fall victim to this virus. Inaugurating what has since become a commonplace tactic, the Kournikova virus enticed email recipients to open an attached picture of the statuesque tennis star.
There was in fact no image behind the message — just an obsessed young programmer from the Netherlands, who quickly turned himself in to authorities.
In 2001, anti-virus researchers were frustrated by a new worm dubbed Code Red, after the hyper-caffeinated flavor of Mountain Dew soda its finders were drinking when they discovered it.
Code Red attacked Microsoft servers and during the summer of 2001 infected more than 350,000 computers. It proved tricky to eradicate because it was able to re-infect cleaned systems, causing overload and denial-of-service problems for sites around the world.
Using a tripartite attack, Nimda ("admin" spelled backwards) was not only a virus (an alteration to a benign program or file) but also a worm and a Trojan horse (a standalone program that pretends to be benign).
Nimda's variety of attack methods enabled it to spread faster than any previous malware, spanning the globe in less than an hour. (It appeared on Sept. 18, 2001, leading to media speculation of an Al Qaeda connection.) Although estimates vary, it is reported to have caused billions of dollars worth of damage.
Netsky and Sasser
By 2004, virus writers were rapidly exploiting and building on each other's code, so much so that they were beginning to interfere with one another. So the Netsky and Sasser worms took the extraordinary step of attempting to clean out other worms on a victim's PC before installing themselves.
Sasser drew attention because it knocked out the satellite communications system for the French news agency Agence France-Presse and caused problems with Delta Air Lines systems, causing some flight cancellations. Eventually, both viruses were traced to a teenage computer science student in Germany.
This story was provided by SecurityNewsDaily, a sister site to LiveScience.