The Most Damaging Computer Viruses Ever

Computer Viruses

Every year, cyber criminals invent ever more sophisticated computer viruses intent on inflicting chaos across thousands of PCs, usually for financial gain. In 2014, CryptoLocker and the “Regin” Trojan made big news, but they still fell some way short of the havoc caused by the most damaging computer viruses of all time.

The Simple Ways in Which Computer Viruses Spread

Understanding Malicious Software and Other Security Threats


An email containing an attachment with the words “I Love You” may have seemed innocent enough, but this virus ended up infecting half a million PCs and costing an estimated $15 billion in damages.

Once the email was opened, the virus stole the user’s image files in the hope of gaining access to online passwords. By automatically sending itself to the first 50 contacts in the victim’s address list, maximum damage was assured.
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Back in 2004, the Mydoom malware made history as the fastest-spreading worm in computer history, ultimately infecting roughly two million PCs.

Mydoom infiltrated the user’s device by appearing as an error message or bounced email, but once opened, spread to anyone in the victim’s Outlook address book. Despite Microsoft offering a $250,000 reward for information concerning the creator of the bug, Mydoom’s author has never been identified.


The Concept virus was an unusual piece of malware as it actually originated not via a malicious email but on an official Microsoft CD-ROM by mistake.

Concept was the first virus to infect Microsoft Word documents, something it utilised to spread rapidly as most documents were shared via email. Although the Concept virus had little noticeable effect on the victim’s computer, it did highlight just how quickly and easily malware can spread.


In 2001, Nimda gained notoriety for its three-pronged method of infection. It could infect the user’s email account and then spread to their email contacts, infiltrate web servers and spread automatically across a network, or infiltrate existing programson a hard disk.

Within just 25 minutes it had infected hundreds of devices, bringing down networks across the globe. The virus hit just a week after the 9/11 attacks, meaning that the Nimda virus increased fears of a new wave of cyberterrorism.

Code Red

Code Red exploited a vulnerability found in Windows 2000 and Windows NT operating systems to devastating effect.

It was able to deface and even temporarily take down a number of websites, including that belonging to the White House. It spread by randomly selecting 100 IP addresses at a time and infecting any vulnerable machines, meaning it wasn’t long before government agencies all over the world were temporarily taking down their own sites to prevent infection.


The Storm virus gets its name from the initial method used to infect PCs. In 2007, potential victims began receiving emails containing news of a deadly storm hitting Europe.

Anyone clicking on the link began unknowingly downloading the virus, leaving them susceptible to hackers intent on stealing personal information. The email used to spread the malware changes its topic to keep up to date with major news stories, but the Storm virus continues to be a major security threat to this day.


The Sobig malware was not only a computer worm but also a Trojan, disguising itself as something other than a virus.

Through the sheer volume of emails sent by infected devices, traffic was brought to a standstill for a number of major companies, including Air Canada and the BBC. By the time the Sobig worm had deactivated itself on 10 September 2003, it had caused $37.1 billion in damages.


Slammer was a computer worm that primarily affected businesses running SQL servers, but the fact that consumer devices were spared didn’t prevent the virus from becoming one of the most damaging ever to exist.

By infecting IP addresses at random, Slammer was able to spread extremely rapidly, ultimately attacking nearly half of all Internet servers. Before order could be restored, Slammer managed to bring down Bank of America’s ATM systems, a nuclear plant in Ohio and Seattle’s 911 emergency services.


The Conficker virus was truly a global phenomenon, infecting millions of computers and causing Microsoft to form an international industry group to counter the threat it posed.

This particular piece of malware utilised a sophisticated method of cracking administrator passwords, making it particularly difficult to remove. Conficker could be used to hijack computers, but in most cases simply prevented new downloads, disrupting a number of major government agencies. The UK’s Ministry of Defence and Germany’s Bundeswehr were affected, while French military aircraft had to remain grounded as flight plans could not be downloaded.


The Melissa virus, named by its creator David Smith after a Miami stripper, was among the first viruses to be spread via email.

The virus dispersed rapidly, primarily because the malicious email appeared to be from someone that the victim knew, ensuring that it quickly overloaded servers and caused an estimated $80 million worth of damage.

Despite Smith claiming that he never intended the virus to cause such widespread harm, he subsequently received a 20-month prison sentence and was banned from going near a computer without court consent.

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