Computer viruses affect people all over the world. But which ones are the worst of all time? The following is a list of some of the most well known viruses and malware to be made public. Most virus protection software (e.g. Norton Antivirus, McAffee, or PC Tools Antivirus) on the market will protect you from these.

Melissa
Combine the illicit thrill of an exotic dancer with the manipulative genius of a hacker and you have one of the worst computer viruses of all time. Melissa was created by David L. Smith, named for his favorite Friday Night Gal, and released into the world on March 26th, 1999. Posing as an email attachment, the self-replicating virus activated when the malicious attachment was opened, then sent itself to the top 50 people in the email client contact list. The damage was so great that some companies had to shut down email programs until the virus was contained. Smith was convicted, fined $5,000 and spent 20 months in jail. Before Melissa, public knowledge of the detriment of malware was previously unknown.

ILOVEYOU/Love Letter
Ironically named, this love letter was sent from the Philippines in May of 2000 and wreaked havoc on computers around the world. Beginning as an email that claimed the attachment contained honey-filled words from a secret admirer, the subsequent worm that was unleashed worked in multiple ways. After copying itself into several different files and adding new registry keys to the victim’s computer, ILOVEYOU would then download a password stealing application that would email personal data to the hacker’s account. ILOVEYOU then used email and chat clients to send itself to other sources, further perpetuating the cycle. Some sources claim the ILOVEYOU computer virus caused over $10 billion in damages.

Code Red
Taking advantage of a vulnerability in Windows 2000 and Windows NT operating systems, the Code Red and Code Red II computer worms began to gain traction shortly after their 2001 release. Creating a large botnet by installing backdoors on infected machines, Code Red initiated a DDos (distributed denial-of-service) attack on the White House by commanding all computers within its extensive network to contact its web servers at one time. This act overloaded the servers, rendering them unable to perform their needed actions.

SQL Slammer/Sapphire
The SQL Slammer, also known as Sapphire, was a computer virus that infected the most heavily used web servers across the US at an alarming rate. In January of 2003, the SQL Slammer caused a number of issues including outages in 911 service in Seattle, crashed the Bank of America’s ATM service, and left Continental Airlines with so many electronic issues that they were forced to cancel flights. Over the course of the computer virus’ extensive life, it caused over $1 billion in damages before antivirus and antispyware software was able to patch the problem.

Sasser/Netsky
A relatively new exploit, Sasser, began to infect computers around the world on April 30th, 2004 by taking advantage of non-updated Windows 2000 and Windows XP operating systems. Once a PC was infected with Sasser, the malware would scan the PC for other unprotected computers in its network and replicate onto them. Aside from causing massive damage to the computer, Sasser also made shutting down any computer difficult without cutting off the power source. The same group of black hat hackers that created Sasser also took credit for Netsky, a virus that propagated through an email attachment, causing massive DDoS attacks. At their height, the two viruses were said to have cost tens of millions of dollars in damage, including forcing flight cancellations and delays for Delta Airlines and shutting down satellite communications a few French news agencies.

MyDoom
MyDoom makes the list for its ability to bring prominent search engines to their knees. In February of 2004, the creators of MyDoom released the first phrase of this virus into the world. The worm installed backdoors on computers and initiated a DoS attack. The worm was commanded to stop distributing just short of two weeks after it began. Later that year, MyDoom was released again with greater voracity. Like other viruses of its lot, MyDoom searched email contacts as a method of proliferating. Unlike other viruses, MyDoom also submitted these contacts as a query to search engines like Google in an unprecedented denial of service attack. With millions of search requests from corrupted computers coming in, search engines were significantly slowed and some even crashed.

Klez
Known as a computer virus that broke ground, Klez goes down in infamy as one of the most malicious viruses of all time. In late 2001, Klez began infecting computers through email messages that would install, replicate and then send themselves to every contact in the infected computer’s address book. Klez also used a tactic called “spoofing” – putting the names of people from the contact list in the “From” line and sending away – giving the impression that the email messages were coming from someone else. The malicious incarnation carried harmful programs that could function like a normal virus, disable antivirus software, or appear as a trojan. The worst forms of the virus rendered infected computers completely inoperable.

Don’t be caught without an antivirus software on your computer. At the least do it for your friends and family. You don’t want to be the person sending a virus that harms their computer or their contacts.