Computer Virus Examples (2024): The 9 Worst Attacks Ever

By Tibor Moes / Updated: May 2024

Computer Virus Examples (2023): The 17 Worst Attacks Ever

Computer viruses pose a significant threat to global digital security, causing widespread damage to personal, corporate, and governmental systems.

In this article, we will explore nine of the most devastating computer virus attacks in history, providing insights into their impact and the lessons learned from each incident.


A computer virus is a type of malicious software that replicates itself by modifying other computer programs and inserting its own code.

  • Morris Worm (1988): This early internet worm rapidly infected around 6,000 computers, showcasing the vulnerability of connected systems. The financial damage soared into the millions, marking a new era in digital security threats.
  • ILOVEYOU (2000): A deceptive email worm that infected over ten million Windows PCs, causing widespread data loss and significant financial damage.
  • Code Red and Code Red II (2001): These worms targeted Microsoft’s IIS web server, infecting over 359,000 systems in less than 14 hours and causing over $2 billion in damages.
  • Mydoom (2004): Known for causing an estimated $38 billion in damages, Mydoom infected around 50 million computers, making it one of the most damaging viruses ever.
  • Stuxnet (2010): A sophisticated worm that targeted Iran’s nuclear program, damaging about one-fifth of its nuclear centrifuges and infecting over 200,000 computers.
  • CryptoLocker (2013): This ransomware infected up to 250,000 computers, with its operators extorting around $3 million from victims.
  • Bad Rabbit (2017): A ransomware attack that demanded payments in Bitcoin, affecting numerous organizations and translating to a ransom of about $1,070 today.
  • WannaCry (2017): Infected about 200,000 computers across 150 countries, with financial and economic losses potentially reaching $4 billion.
  • Shlayer (2018): The most common threat for macOS users in 2019, affecting 10% of all Macs analyzed by Kaspersky.

Don’t become a victim of a computer virus. Protect your devices with the best antivirus software and your privacy with the best VPN service.

Computer Virus Examples

1. Morris Worm (1988)

In the late 1980s, the digital world witnessed one of its first major security crises with the emergence of the Morris Worm. This incident, a landmark in the history of cyber threats, unfolded rapidly and with startling impact. Within just 24 hours of its release, the Morris Worm had infiltrated an estimated 6,000 computers. This number might seem modest by today’s standards, but it was a significant proportion of the roughly 60,000 computers connected to the then-nascent Internet.

The worm, named after its creator Robert Tappan Morris, was not designed with malicious intent. Instead, it was an experiment that spiraled out of control. Morris, a graduate student at Cornell University, aimed to gauge the size of the Internet. However, due to a programming error, the worm replicated excessively, clogging up network resources and slowing down systems to the point of unresponsiveness.

The financial toll of the Morris Worm was substantial, though difficult to quantify precisely. Initial estimates of the damage started at around $100,000, but as the full extent of the disruption became clear, figures soared into the millions. The incident served as a wake-up call to the digital world, highlighting the need for better security protocols and measures. It was a stark reminder of how quickly and extensively digital threats could propagate, even in the Internet’s early days.

2. ILOVEYOU (2000)

Fast forward to the year 2000, and the world faced a new, more sophisticated breed of computer worm: ILOVEYOU. This deceptively named worm unleashed a global crisis, infecting over ten million Windows personal computers starting on May 5, 2000.

The ILOVEYOU worm was a masterclass in social engineering, exploiting human curiosity and trust. It spread via email with the subject line “ILOVEYOU” and an attachment named “LOVE-LETTER-FOR-YOU.txt.vbs.” Many recipients, intrigued by the romantic connotation, opened the attachment, unwittingly unleashing the worm.

Once activated, ILOVEYOU replicated itself and overwrote files, causing widespread data loss. It also emailed itself to the first 50 contacts in the user’s Windows address book, which significantly aided its rapid global spread. The financial impact of ILOVEYOU was staggering, with damages estimated in billions of dollars. It affected both individual users and large organizations, including corporations and government agencies.

The ILOVEYOU worm was a significant event in cybersecurity history, illustrating the devastating potential of computer viruses and the importance of cautious online behavior. It underscored the need for robust digital hygiene practices, such as not opening suspicious emails and attachments, and the critical role of up-to-date security software.

3. Code Red and Code Red II (2001)

In the summer of 2001, the digital world was shaken by the emergence of Code Red, a rapidly spreading computer worm. This cyber threat was remarkable for its speed and scale of infection. Within a mere 14 hours of its release, Code Red had successfully infiltrated over 359,000 systems. The worm targeted computers running Microsoft’s IIS web server software, exploiting a known vulnerability to replicate and spread.

Code Red was unique in its approach. It didn’t require human interaction, such as opening an email or a file, to propagate. Instead, it automatically infected vulnerable systems connected to the Internet. The worm caused widespread outages and significantly slowed down Internet traffic, affecting both individual users and large organizations.

The financial repercussions were enormous, with the total damage caused by Code Red estimated to exceed $2 billion. This figure encapsulates the costs of disrupted services, lost productivity, and the extensive efforts required to eradicate the worm from infected systems.

The subsequent variant, Code Red II, emerged shortly after and used a similar method to spread. It further highlighted the need for timely software updates and robust cybersecurity measures to protect against such vulnerabilities.

4. Mydoom (2004)

Mydoom, which struck in 2004, stands as one of the most damaging viruses in history. Its impact was both widespread and costly, with estimates suggesting that Mydoom caused a staggering $38 billion in damages. This figure reflects the extensive harm and disruption caused to businesses, governments, and individuals across the globe.

The reach of Mydoom was unprecedented, infecting around 50 million computers worldwide. It spread primarily through email, enticing recipients with deceptive subject lines and attachments. Once opened, the virus replicated and sent itself to email addresses found in the user’s contact list, perpetuating a rapid and extensive spread.

Mydoom’s design was not just to replicate; it also opened backdoors on infected computers, leaving them vulnerable to further exploitation. This aspect of the virus caused long-term security concerns, as it allowed attackers to access and control infected systems even after the initial outbreak was contained.

The Mydoom virus underscored the importance of cautious email practices and the necessity of having updated antivirus software. It also highlighted the broader implications of such attacks on Internet infrastructure and the global economy.

5. Stuxnet (2010)

In 2010, the cybersecurity world was rocked by the discovery of Stuxnet, a highly sophisticated computer worm unlike any seen before. Stuxnet was not just a piece of malware; it was a weaponized software designed to target and disrupt specific industrial control systems. The primary target of this cyberattack was Iran’s nuclear program, where it reportedly damaged almost one-fifth of the country’s nuclear centrifuges.

The scale and precision of Stuxnet were unprecedented. It infected over 200,000 computers, but its real impact was in the physical world – causing 1,000 machines to physically degrade. This worm marked a significant shift in the landscape of cyber threats, moving beyond data theft and vandalism to the realm of physical destruction. Stuxnet operated by exploiting multiple zero-day vulnerabilities and was capable of stealthily manipulating industrial processes.

The revelation of Stuxnet’s capabilities and targets sent shockwaves through the international community. It highlighted the potential for cyber warfare to cause physical damage to critical infrastructure, a concern that extended far beyond Iran’s borders. This incident underscored the need for stringent security measures in industrial control systems and raised questions about the ethics and implications of state-sponsored cyber warfare.

6. CryptoLocker (2013)

CryptoLocker, which emerged in 2013, represented a new and insidious threat in the form of ransomware. This malicious software encrypted the files on a victim’s computer, then demanded a ransom for the decryption key. By mid-December of that year, Dell Secureworks reported that between 200,000 to 250,000 computers had been infected by CryptoLocker.

The impact of CryptoLocker was not just in its widespread infection but in the financial losses it inflicted. The operators of this trojan were alarmingly successful, extorting an estimated total of around $3 million from victims. The ransom typically had to be paid in Bitcoin, making the transactions difficult to trace and prosecute.

CryptoLocker’s modus operandi opened a new chapter in cybercrime, where attackers could directly monetize their exploits through victim payments. It highlighted the importance of regular data backups and the dangers of opening email attachments from unknown sources. The CryptoLocker saga also prompted a significant response from law enforcement and cybersecurity professionals, leading to a coordinated effort to take down the network used by the malware.

7. Bad Rabbit (2017)

In 2017, the digital world was hit by another ransomware attack known as Bad Rabbit. This malware, as reported by, demanded a ransom of 0.05 Bitcoins, equivalent to about $290 at that time. Given the fluctuations in Bitcoin’s value, this amount would be approximately $1,070 today.

Bad Rabbit spread through a fake Adobe Flash update on compromised websites, tricking users into installing it. Once installed, it encrypted the user’s files, displaying a ransom note with the payment demand. The attack primarily targeted organizations in Russia and Ukraine but also affected systems in Turkey, Germany, and other countries.

Bad Rabbit’s method of spreading and its financial demands highlighted the evolving tactics of cybercriminals and the importance of being cautious with software updates and downloads from the internet.

8. WannaCry (2017)

WannaCry, another ransomware attack that occurred in 2017, was unprecedented in its scale and impact. According to Europol and as reported by, around 200,000 computers were infected across 150 countries, making it one of the most widespread cyberattacks in history.

The financial and economic losses from the WannaCry attack were immense, potentially reaching up to $4 billion. WannaCry exploited a vulnerability in Microsoft Windows, and despite a patch being available, many systems remained unupdated and vulnerable. The attack caused significant disruptions in various sectors, including healthcare, where it infamously impacted the UK’s National Health Service.

WannaCry’s global reach and the substantial financial losses it caused underscored the critical need for regular software updates and robust cybersecurity measures in both personal and organizational contexts.

9. Shlayer (2018)

In 2018, the cybersecurity landscape for macOS users changed dramatically with the emergence of the Shlayer Trojan. This malware challenged the long-standing perception that Mac systems were largely immune to the kind of security threats frequently encountered on Windows platforms.

As reported by, by 2019, the Shlayer Trojan had become the most common threat for macOS users. A startling statistic from Kaspersky revealed that 10% of all Macs analyzed were affected by this Trojan, underscoring its widespread impact.

Shlayer was particularly insidious because it disguised itself as a legitimate software update or download, tricking users into installing it. Once installed, it bombarded users with an onslaught of adware and potentially unwanted programs. This not only compromised the user experience but also exposed users to further security risks.

The prevalence of Shlayer served as a critical wake-up call to the Mac community, highlighting the importance of vigilance even in a relatively secure ecosystem. It reminded users and developers alike that no system is impervious to attack and that maintaining cybersecurity is a continuous and evolving challenge.


The history of computer viruses, from the Morris Worm to Shlayer, illustrates a clear and evolving threat in the digital world. These examples highlight not only the sophistication and diversity of cyberattacks but also the extensive financial and operational damages they can inflict. Each incident serves as a reminder of the vulnerabilities inherent in digital systems and the continuous need for vigilance and updated security measures.

In the context of these threats, particularly for users of Windows 11, investing in robust antivirus software is more crucial than ever. Brands like Norton, Avast, TotalAV, Bitdefender, McAfee, Panda, and Avira offer comprehensive protection that goes beyond basic defense mechanisms. These antivirus solutions provide real-time monitoring, advanced threat detection, and proactive measures to prevent malware infections.

With cyber threats becoming more sophisticated, the right antivirus software is not just a tool—it’s an essential layer of defense that protects personal data, financial information, and ensures uninterrupted digital experiences. The cost of antivirus software pales in comparison to the potential losses from a cyberattack, making it a wise and necessary investment for any Windows 11 user.




Author: Tibor Moes

Author: Tibor Moes

Founder & Chief Editor at SoftwareLab

Tibor has tested 39 antivirus programs and 30 VPN services, and holds a Cybersecurity Graduate Certificate from Stanford University.

He uses Norton to protect his devices, CyberGhost for his privacy, and Dashlane for his passwords.

You can find him on LinkedIn or contact him here.