The Worst Computer Viruses & What They Did To The Internet

Computer viruses can be vicious little blighters wreaking all sorts of damage and destruction; destroying files, corrupting hard drives, plundering personal details and slowing the Internet to a crawl. Since the dawn of computing for the masses countless viruses have been released. Today, an estimated one million viruses are in circulation and thousands are created every month.

Computer viruses don’t just slow your computer down and annoy you with popups. Viruses can also steal your credit card information, hold your data hostage and even wipe your hard drive clean. Here’s a look at some of the worst viruses in history.


Created in 1999 by David L. Smith

The Virus

The virus was a Microsoft Word Macro. A Macro is a series of commands or instructions that gets carried out automatically. He claimed to have named the virus after an exotic dancer in Florida.

One of the first email – activated viruses.

It affected users with Microsoft Word 97 and 2000 by:

Lowering security settings.

Shutting down safeguards in those programs.

The virus spread itself by sending an infected document via email.

Disabling macro security.

The email was designed to trick people into opening the file, computers which had Microsoft Outlook would send the infected document to the top 50 contacts in the users’ address book. If the day of the month matched the minute, the virus would insert a Bart Simpson quote into the documents it sent:

“Twenty-two points, plus triple-word score, plus fifty points for using all my letters. Game’s over. I’m outta here.”

The Damage

  • Tens of thousands of people couldn’t access their emails within 6 hours of the virus being posted.
  • Hundreds of websites were affected.
  • The Microsoft Corporation had to disable all incoming and outgoing emails.
  • Caused £1.2 billion in damages and losses.

David L. Smith was:

Fined over £3,000.

Sentenced to 20 months in jail.

Forbidden from accessing computer networks without court authorisation.


Allegedly written by Onel de Guzman

The Virus

Typically spread through an infected email attachment.
Launched from the Philippines in 2000, the email subject line would say that it was a love letter from a secret admirer. The name of the original file was “LOVE-LETTER-FOR-YOU.TXT.vbs”

Due to formatting issues, some email clients omitted the “.vbs” in the file name. This caused users to think they were opening a plain text file.

The Virus Would

Overwrite file types with copies of itself to let it continue spreading if the original version was removed from the computer.

Reset the infected computer’s Internet Explorer homepage.

Download & execute a file that stole passwords & emailed them to the hacker’s email address.

Send the infected file to all of the user’s contacts in Microsoft Outlook.

The Damage

  • Roughly one tenth of all Internet-connected computers in 2000 were infected with ILOVEYOU.
  • Day 1: ILOVEYOU reached an estimated 45 million people.
  • Day 7: It caused £3 billion in damages in the first week.
  • The virus in total caused an estimated £11 million in damages.
  • McAfee reported that a super majority of their future 100 clients were infected with the virus.

Onel de Guzman was:

Arrested on suspicion of creating the virus.

He & his co-conspirator were later released as the Philippines had no laws at the time against writing malware.

Code Red

Code Red launched in July 2001, a second version of the virus, Code Red II, acted similarly and was launched later in the year.

The Virus

Works by sending the computer instructions after a long string of nonsense.

Once the buffer had been filled with the nonsense information, the computer begins overwriting memory.

This meant that the user only had to be connected to the Internet to be infected.

Infected Windows NT machines would crash more often than normal.

The Damages

  • Between 1 and 2 million computers were infected overall.
  • In less than a day, the virus infected more than 359,000 computer system.
  • Caused over £2 billion in losses.


Launched in September 2001, one week after 9/11.

The Virus

The FBI had to refute rumours that the virus was connected to the terrorist attack.
In Computer Magazine, TruSecure CTO Peter Tippet reported that Nimda topped their list in just 22 minutes. The virus was the fastest spreading piece of Malware at the time and more than 2 million computers were infected in just 24 hours.

While the virus could infect home PC’s its primary targets were web servers.

The virus infected computers in many ways

Local Network.

Drive-by Downloads on websites.

Loopholes created by other worms.

Vulnerabilities in IIS, Microsoft web server.


Slammer / Sapphire

Launched in 2003.

The Virus

Spread through a buffer overflow vulnerability in Microsoft’s SQL server database management service.
Randomly selected IP addresses to infect.
Servers infected with SQL Slammer would spawn millions of copies to infect other servers.
Within 3 minutes of attacking its first victim, the number of servers infected by Slammer doubled every 8.5 seconds.

The Damages

  • Caused $750 million in damages.
  • Crashed Bank of America’s ATM service.
  • Infected Continental Airlines online ticketing systems and electronic kiosks, rendering them inoperable.
  • South Korea lost almost all internet access.


Launched in 2004. Created by Sven Jaschan, a 17 year old from Germany

The Virus

Sasser worked by exploiting a vulnerability in a Windows system called LSASS (Local Security Authority Subsystem Service. The virus scanned IP addresses until it found one that was vulnerable. Then, it downloaded itself into the Windows directory so that the next time the computer was booted up it would be infected.

Sasser also affected operating systems, this made shutting down an infected computer without pulling the plug difficult. Unlike other viruses, users didn’t have to open any email attachments in order to be infected by Sasser. They only needed to be online.

The Damages

  • Sasser brought down a third of Taiwan’s post offices.
  • Infected all 19 of the British Coastguard’s control rooms.
  • Delayed British Airways’ flights.
  • Staff had to use paper maps and pens.

Sven Jaschan was sentenced to:

1 year, 9 months probation.

30 hours of community service.

He was tried as a junior.


The Virus

Originally began spreading through KaZaA, a file sharing application, but then spread to emails. In both cases, users had to open a file in order to become infected.

As it’s peak, MyDoom infected on in 12 emails as it tried to spread itself. The virus would open parts on a victims’ computer so that hackers would have backdoor access to their systems.

A second attack later that year affected search engines, MyDoom was capable of spoofing its infection emails, making it more difficult to track.

MyDoom infected between 600,000 and 700,000 computers.

The Damages

  • Caused $38 billion in damages.
  • Slowed down internet access worldwide by 10%.
  • Reduced access to some websites by as much as 50%.


Launched in 2008

The Virus

Took advantage of an exploit in Windows 2000, XP, and 2003 servers that could case them to install an unauthenticated file. It could even affect servers with firewalls, as long as they had print and file sharing enabled.

Conficker infected millions of computers by infected USB drives and over networks. Later variants were capable of, creating backdoor in firewalls and disabling anti-malware programs.

The Damages

  • Caused $9.1 billion in damage.
  • French fighter planes were grounded when they couldn’t download their flight plans.
  • In England military systems were infected, including: more than two dozen British Royal Air Force bases.
  • 75% of the royal Navy fleet.
  • Computers and medical devices at hospitals in the US and UK were infected.

Worst Computer Viruses 1