The 15 Most Dangerous Computer Viruses of All Time Wreaked Serious Havoc

types of cybercrime

The 15 Most Dangerous Computer Viruses of All Time Wreaked Serious Havoc

Key Points

  • Stuxnet, created by the US and Israel, physically damaged Iran’s nuclear facilities, showcasing the real-world impact of computer viruses.
  • CryptoLocker introduced ransomware and caused around three million dollars in losses.
  • Sasser spread across millions of computers without user interaction, causing system crashes and million-dollar damages.
  • Storm Worm created a versatile botnet capable of spamming, stealing information, and launching DDoS attacks, causing millions of dollars in damage.
  • Blaster exploited a Windows vulnerability and was partially developed by an 18-year-old, causing $50 million in damages.

Throughout the history of computing, certain malevolent programs have risen to notoriety for wreaking havoc on digital landscapes and leaving a trail of chaos in their wake. These viruses are characterized by their malicious intent unleashed on unsuspecting online victims. Each has posed threats to individuals, corporations, and even nations, causing data loss, financial losses, and widespread disruption across the board. Let’s take a look at the 15 most dangerous computer viruses in history below.

What Are Computer Viruses?

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Computer viruses cause billions in damage every year.


A computer virus is like a digital infection that thrives on computers or other online networks. They work by either secretly exploiting weaknesses in software or manipulating users into activating them. Just as a virus in the human body will spread, infect cells, and make a person sick, computer viruses infect and spread across computer files, programs, and systems. Here’s a breakdown of how they operate.

First, a virus enters your computer by attaching itself to innocent-looking files or programs. Once inside, it starts making copies of its infected code. These copies then spread to other files, programs, and even other computers through shared files, emails, or network connections. 

Viruses don’t always cause damage immediately. Instead, some wait for a trigger. Once activated, the virus then goes to work. Similarly, damage can also vary. Some might corrupt or erase your files, while others steal personal information or even take control of your computer. 

To spread, viruses may attach themselves to emails and send themselves to everyone in your contact list. Or, they might exploit vulnerabilities in your software to hop onto other devices connected to the same network. Viruses try to hide themselves by altering their code or using encryption to evade detection by your device’s antivirus software. 

To protect yourself against viruses, keep your operating system and other software up to date. Strong, unique passwords are another surefire way to protect your data. As a rule of thumb, be cautious with unfamiliar email attachments or links and avoid downloading files from sketchy websites.

The Most Dangerous Computer Viruses Ever

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Computer viruses are nothing more than malevolent code, but they cause catastrophic damage.

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These 15 infamous computer viruses stand as stark reminders of the importance of cybersecurity. Each one played a major role in the continuous evolution of online defenses. All 15 computer viruses pushed the cybersecurity industry to do a better job safeguarding our digital world from such malevolent threats in the future. What were their methods, their impacts, and, most importantly, the lessons they taught? Let’s break all 15 down below.

15. Stuxnet (August 2010)

Interestingly enough, Stuxnet is said to have been created through a collaboration between the United States and Israel. While neither country has ever admitted to this, Stuxnet was distinctively designed for cyber espionage. Once unleashed, the virus targeted industrial control systems within Iran’s nuclear facilities. By subtly altering the operating speeds of centrifuges, it physically damaged equipment, showcasing the potential for computer viruses to impact real-world conflicts.

14. CryptoLocker (September 2013)

This virus marked a turning point in cybercrime by introducing ransomware to the mainstream. In short, CryptoLocker encrypted its victims’ files and demanded payment in exchange for the decryption keys. With this relatively simple act, the virus demonstrated the sheer profit potential of exploiting individuals’ data and privacy. Estimates put total losses at around three million dollars between September 2013 and May 2014.

13. Sasser (April 2004)

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Microsoft operating systems have been at the center of many major virus scandals over the years.

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The Sasser virus capitalized on a flaw in Windows XP and Windows 2000 operating systems. Through this vulnerability in the systems, Sasser was able to spread itself across millions of computers without any user interaction whatsoever. That’s what made it so dangerous. It caused frequent system crashes and slowdowns, disrupting operations and necessitating significant efforts to patch and clean infected systems. Total damages were in the million-dollar range.

12. Storm Worm (January 2007)

Storm Worm went by many different names during its short life cycle, but one thing remained constant: the millions of dollars in damage done. Disguised as enticing news headlines or e-card notifications, Storm Worm aimed to create a vast botnet capable of executing various cybercrimes. This versatile botnet could send spam emails, steal sensitive information, and even launch DDoS attacks, showcasing its potential for multifaceted digital malevolence.

11. Blaster (August 2003)

Also known as MSBlast, the Blaster virus exploited a Windows vulnerability to create a worm that targeted vulnerable computers running Windows XP or Windows 2000. Once a computer was infected, these machines experienced frequent crashes and could be remotely controlled by hackers. As it turned out, it was at least partially developed by an 18-year-old. Its $50 million in damages emphasized the need for timely patching and proactive security measures.

10. Melissa (March 1999)

Spam folder in email inbox.
Melissa showed the world that computer viruses could be hiding in an email.

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The Melissa virus was one of the earliest instances of a virus spreading rapidly through email. Delivered through infected Microsoft Word documents, the virus replicated itself and overwhelmed email systems upon activation. It caused widespread disruptions, massive data corruption, and even system crashes. The impact was so significant that many organizations had to shut down their email servers to contain the outbreak. It still managed to cause more than $80 million in damages.

9. Zeus (July 2007)

Operating as a banking trojan, the Zeus virus aimed to steal valuable financial information from infected devices. Utilizing tactics like phishing and keylogging, it targeted online banking credentials, credit card details, and any other sensitive data it could get its hands on. Over several years, Zeus posed a significant threat to individuals’ financial security. Total damages exceeded $100 million.

8. CIH (April 1998)

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The Chornobyl virus sought to render all infected computers completely useless.


CIH, also known as the Chornobyl virus or the Spacefiller virus, is the oldest of all the computer viruses listed here. Once the virus was set off on a user’s computer, it would proceed to overwrite critical system data and render the infected computer inoperable. Because of the severity of the virus, CIH caused more than a quarter of a billion dollars in financial losses. This large total came from the need for physical repairs and data recovery.

7. SQL Slammer (January 2003)

The early 2000s were hard for the burgeoning cybersecurity industry. The SQL Slammer is one such example of this. Notorious for its speed, Slammer exploited a Microsoft SQL Server vulnerability and used it to spread throughout the Internet. By generating a large number of random IP addresses, the virus overwhelmed network connections. Thus, Slammer led to major service disruptions and widespread congestion online. Experts estimate the damage was over a billion dollars in all.

6. Nimda (September 2001)

The Nimda virus exploited mass fear surrounding the September 11th attacks to wreak its havoc. Nimda leveraged various means of propagation that included email attachments, web browsing, network shares, and more. This aggressive multi-pronged approach enabled Nimda to spread at an unprecedented pace, congesting networks and impacting both individuals and organizations so soon after the national tragedy. Total damages exceeded a billion dollars.

5. Code Red (July 2001)

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The Code Red virus was named after the soda cybersecurity experts were drinking when they discovered the virus.

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Computer viruses did some serious damage in 2001. As if Nimda wasn’t bad enough, Code Red was unleashed on the world shortly before. Targeting Microsoft IIS web servers, Code Red exploited a vulnerability that allowed it to spread rapidly and replicate itself. It didn’t just compromise servers, though. Code Red also plastered websites with the message “Hacked by Chinese!” — something cybersecurity experts took as a hint about the virus’s origins. Damages exceeded two billion dollars.

4. WannaCry (May 2017)

Leveraging a previously unknown Windows vulnerability, WannaCry spread globally within hours of its creation. This allowed it to do more than four billion dollars in damage before being shut down. Like the CryptoLocker virus of 2013, WannaCry worked by encrypting users’ files and demanding ransom payments in cryptocurrency. Its massive impact underscored the urgency of security patching and further demonstrated the destructive capabilities of modern ransomware.

3. Conflicker (November 2008)

DDoS network attack
DDoS attacks, or distributed denial-of-service, seek to overwhelm networks.

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Operating with stealthy sophistication, Conficker exploited vulnerabilities in Microsoft Windows systems. By leveraging these weaknesses, it established a massive botnet capable of executing various cybercrimes. Conflicker used its network of compromised computers to steal sensitive information, launch distributed denial-of-service (DDoS) attacks, and more to paralyze over nine billion dollars in websites and services.

2. ILOVEYOU (May 2000)

Dubbed the “Love Bug,” ILOVEYOU was a self-propagating worm that spread via email attachments. Masked as a love letter, recipients were lured into opening the attachment. This then activated the virus and caused it to replicate itself to all contacts in the victim’s address book. This fast-spreading virus rapidly clogged email servers worldwide and led to substantial financial damages. The total was estimated at more than 15 billion dollars due to lost productivity, data recovery, and system repair.

1. Mydoom (January 2004)

Of all the computer viruses listed here, Mydoom is far and away the most dangerous of all time. It did a staggering $38 billion in damage in various forms between 2004 and 2009. Typically arriving as email attachments or malicious links, Mydoom aimed to create backdoors in infected systems. Hackers would then use these backdoors to control systems remotely. From there, hackers launched DDoS attacks on specific websites, causing severe traffic congestion and rendering these platforms inaccessible to legitimate users. Its damage has yet to be outdone to this day.

Frequently Asked Questions

What is a computer virus?

A computer virus is a harmful or dangerous program that infects your computer or network. It spreads by attaching itself to files or programs. Viruses have been known to delete files, steal personal information, or slow down your computer or network. They cause billions in damages every year.

How do computer viruses spread?

Computer viruses spread through infected files, emails, or networks. They tend to hide in innocent-looking attachments that trick users into clicking them. Then, when someone opens one of these files or clicks on an attached link, the virus makes its move. Once inside, it starts making copies of itself and spreads to other devices connected to your computer or network.

How do you avoid computer viruses?

You can protect your devices from viruses by using antivirus software, keeping your operating system and software updated, using strong and unique passwords, being cautious with email attachments and links, and not downloading files from untrusted sources. Any one of these tactics will add an extra layer of protection, but your best bet is to use a combination of the bunch.

Can you get a virus from visiting a website?

Yes, it’s possible to get a virus by visiting a compromised website. Some computer viruses are specifically designed to exploit vulnerabilities in your web browser or plugins. To stay safe, make sure your browser and plugins are up to date, and consider using an ad blocker and security software. Also, pay attention when your browser flags a site — it’s doing so for your protection.

What should you do when your computer gets a virus?

If you fear your computer may have been infected by a virus, disconnect from the Internet immediately to prevent further spread. Then, run a full scan with your antivirus software of choice and follow its instructions to remove the virus. If the problem persists, seek help from a professional or use a trusted malware removal tool to get rid of the virus for good.

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