The 10 Most Dangerous Computer Viruses Ranked and Explained

The most dangerous computer viruses can create havoc for users and cause damage that costs billions of dollars to repair. Viruses are defined by their ability to rapidly spread and took center stage in the late 1980s and 1990s with the explosive growth of networked systems and the internet, enabling them to reach millions of machines around the world.

They’re also an enduring concern for computer users, with their ability to disable systems, corrupt data, and enable hackers to steal your details and defraud you. In this guide, I’ll explore the ten most dangerous computer virus outbreaks in history, how they worked, and the damage they caused as they infected millions of devices.

The best protection against the worst virus infections is to use antivirus software, of course. I have extensive experience in testing leading security products, and I’ll also explore how you can protect yourself against the most dangerous virus computer infections that you’re likely to face today.

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Table of Contents Table of Contents

Top 10 Most Dangerous Computer Viruses
How Do the Most Dangerous Computer Virus Infections Work, and How Do They Spread?
How to Protect Yourself from the Deadliest Computer Viruses
Conclusion – The Most Destructive Computer Viruses in History
FAQs
References

Table of Contents

Top 10 Most Dangerous Computer Viruses
How Do the Most Dangerous Computer Virus Infections Work, and How Do They Spread?
How to Protect Yourself from the Deadliest Computer Viruses

Show Full Guide

Conclusion – The Most Destructive Computer Viruses in History
FAQs
References

Top 10 Most Dangerous Computer Viruses

Given the huge amount of damage computer virus infections can cause, it’s very important to have the best antivirus software in place to protect your devices and data. Here’s my full breakdown of the ten most dangerous computer viruses in history.

1. WannaCry

The WannaCry ransomware spread rapidly in May 2017. Once it accessed a system, it encrypted files, making them inaccessible to the device’s owner, and displayed a ransom demand for decrypting the files. The ransom was initially $300, which later increased to $600.

The ransomware exploited a vulnerability in the Windows operating system and attacked computers that hadn’t installed a security patch to combat EternalBlue – a hacking technique devised by the NSA –released by Microsoft two months before the attack.

WannaCry is estimated to have infected around 230,000 systems worldwide. One of the most prominent victims was the National Health Service in the UK. The attack led to the cancelation of almost 19,000 appointments and cost around £92 million to repair the damage. Total global losses are estimated at around $4 billion.

2. CryptoLocker

CryptoLocker ransomware, which first emerged in 2013, works in a similar way to WannaCry, which can be considered a CryptoLocker variant. This dangerous ransomware arrives as an email attachment and encrypts all the files it can find once on your system.

The ransomware uses asymmetrical encryption to lock system files, which can only be decrypted using the attacker’s private key. CryptoLocker can access files on hard drives, USB drives, and even cloud storage, making it all the more dangerous. The hackers then demand a ransom from the victim to release the files.

Like Sasser, CryptoLocker can be blocked with strong antiviruses and good security practices – regular scans and backups go a long way in protecting your system from malicious actors.

3. Melissa

Melissa first appeared in March 1999 when programmer David Smith accessed an AOL account and posted a file named “alt.sex”. The file was promised to contain free passwords and access to several adult websites.

However, when opened in Word, the virus spread through the victim’s system, accessed Microsoft Outlook, and sent emails to the first 50 contacts it could find.

These emails included an attachment containing the virus, often named “sexxxxy.jpg”, and came with messages like, “Here is the document you requested… don’t show anyone else”. If a victim opened the attachment, the whole cycle would repeat.

It cost around $80 million in 1999 to repair the damage done by the virus. When adjusted for inflation, this comes to around $147 million today. The virus infected over 300 corporations and government organizations, including Microsoft and the United States Marine Corps.

4. ILOVEYOU

The ILOVEYOU virus created havoc when it was released in May 2000. It works in a similar way to Melissa, and victims received an email with the subject line, “I Love You.” When victims opened an attachment included with the message, it downloaded a virus, which accessed the host’s Outlook contacts and sent the same email to all the contacts it could find.

The worm was a VBScript program disguised as a text file. Once opened, it overwrote and corrupted files on the system and forwarded login details to the creator of the virus. An i love you virus download could cause major system issues for victims, and given its rapid distribution, it was one of the most dangerous computer virus infections ever seen.

Within just ten days, ILOVEYOU infected close to 50 million computers, and $15 billion was spent on removing the worm from systems around the world.

The virus was so widespread that the Pentagon, the Danish and U.K. parliaments, the U.S. Army, Ford Motors, and the CIA all suspended their email systems to control the damage.

5. Mydoom

Mydoom was a computer worm developed in the early 2000s, which spread through email attachments, and it’s one of the costliest computer viruses ever seen. Once a recipient opened an email containing Mydoom, it would infect the host and send out emails to all the contacts it could find.

Mydoom created a backdoor for cybercriminals to access the infected device. Typically, these hacked machines would be used for DDoS attacks, bombarding websites with traffic and making them inaccessible to regular users. It’s rated as potentially the most dangerous virus in computer history.

One of Mydoom’s targets was SCO Group, and a significant fraction of systems infected with the worm were used to shut down the company’s servers. The virus also hit several Microsoft websites and blocked access to antivirus websites, slowing down efforts to clean systems and remove the malware.

Mydoom infected almost 500,000 computers within a week and ultimately resulted in at least $38.5 billion of damage. Given this cost, it’s often cited as the answer to the question, “What is the worst computer virus ever made?”

6. Sobig

Sobig surfaced in 2003. It was a Trojan horse and a worm that spread through emails containing a .pif file. Earlier versions of Sobig were relatively harmless, but the Sobig.F variant crippled systems worldwide.

Once the .pif file was downloaded onto a system, the Sobig virus spread to all connected local networks and hard drives. It then found stored emails and re-sent them with the attached .pif file.

Sobig spread so widely that one in every 17 emails contained the Sobig worm at one point. A 2018 study put Sobig second only to Mydoom when it came to the speed of its distribution.

Some prominent victims of the virus included the BBC and Air Canada. Estimated damages caused by the worm stand at an enormous $35 billion.

7. Klez

The Klez virus first appeared in October 2001. It accesses the victim’s contact book and spreads through emails that include an infected attachment. It was more dangerous than other viruses, however, due to the fact that it was “polymorphic” and could change its code to avoid detection.

One notable feature was that Klez could alter the “From” field in emails, making it harder to track what devices were infected and where emails had been sent from.

In 2001, Klez spread to almost 7% of all PCs worldwide, causing an estimated $20 billion of damage.

8. Code Red

Code Red was a worm discovered by eEye Digital Security in 2001 that used vulnerabilities in Microsoft’s Internet Information Server (IIS).

The virus kept a monthly schedule. From the 1st to the 19th of each month, the worm looked for vulnerable systems on the internet to infect.

From the 20th to the 27th, it then carried out DDoS attacks on official US government websites, bombarding them with spam traffic until they crashed. Once hacked, the websites displayed a message reading, “Hacked by Chinese.”

Code Red is said to have affected more than 30,000 organizations and 300,000 servers worldwide, causing $2.75 billion of damage.

9. Sasser

Released in 2004, Sasser took advantage of unpatched Windows computers and exploited a buffer overflow known as LSASS, hence the name Sasser. Sasser attacks frequently resulted in random system shutdowns with LSASS.exe messages.

While Sasser was a dangerous virus, it was relatively easy to stop. Up-to-date systems and firewalls could easily detect and stop Sasser. However, poor defenses and a lack of computer awareness helped Sasser spread.

AFP was one of the major victims of the virus and had to shut down its operation for several hours. Similarly, Delta Airlines had to cancel several flights after a Sasser attack.

10. Zeus

Zeus, first seen in 2007, is a dangerous virus used by hackers to steal victims’ financial and banking credentials through keylogging and website monitoring. The virus can recognize when a user is on a banking website and then starts recording keystrokes to steal passwords.

The virus also added infected computers to botnets, connecting them to networks of infected machines controlled by hackers without the owner’s knowledge.

The virus spread through spam messages containing a malicious link. When clicked, this automatically installed malware. The hackers also infected popular websites, which then spread the malware.

Zeus wreaked havoc in 2011 when its code was made public. Prominent victims included Bank of America, Amazon, Monster.com, and Cisco. Given its ability to steal financial data, it’s perhaps the most dangerous virus on this list.

How Do the Most Dangerous Computer Virus Infections Work, and How Do They Spread?

Computer viruses include malicious programs and code injected into systems to sabotage their operations. They can cause a large amount of damage, from erasing data on hard drives to unexpectedly crashing systems, resulting in total data loss.

Once activated, viruses typically self-replicate, often creating altered copies to avoid detection, and then set about attacking the host system. People often now use the terms “viruses” and “malware” interchangeably and describe any piece of malicious software as a virus, whether or not it is designed to spread virally.

Historically, viruses were geared toward disrupting systems, but modern malware is generally used by cybercriminals to hack accounts, steal bandwidth and processing power, and commit fraud and identity theft.

Emails and malicious websites are the most common attack points for viruses today. In phishing attacks, fraudsters will attempt to trick you into handing over your login details, potentially entering them into fields on a fake site, or will expose you to malware via attachments and infected landing pages.

These kinds of attacks can also expose you to spyware, which can enable hackers to take complete control of your system. This includes being able to log your keystrokes, track your location, use your camera and microphone to spy on you, and even wipe and disable your device. Given these capabilities, spyware is one of the most pernicious types of computer virus.

Ransomware attacks are another major threat, and see hackers encrypting systems and holding devices and data to ransom. This is devastating for individuals but can be even more costly for businesses. Organizations that are reliant on their systems and data but which have poor network security are particularly vulnerable to this type of malware.

Fortunately, you can protect yourself from the most harmful computer virus attacks and banking malware attacks with antivirus software.

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How to Protect Yourself from the Deadliest Computer Viruses

There are various steps you can take to protect yourself from even the deadliest computer virus. Here are my recommendations.

Get an Antivirus – Modern antivirus solutions from providers like TotalAV and Bitdefender scan your system in real time to detect unusual behavior from files and programs and check emails, downloads, and links for potential threats. Many plans also include features to combat phishing and fortify your system against ransomware and spyware infiltration.
Use a VPN – VPNs encrypt and anonymize your connection, protecting you from eavesdropping by hackers and advertisers. They’re particularly useful when using public Wi-Fi and can stop malicious individuals from intercepting your traffic in Man-in-the-Middle attacks and using the information to hack your accounts.
Use Strong Passwords and MFA – It’s important to avoid common passwords, as these can be easily guessed, and to use different passwords for every account to stop a single data breach from exposing all your online accounts. It’s also important to use multi-factor authentication (MFA) wherever possible, so that even if your login details are stolen, you still have some protection. If this sounds like a pain then the best password managers fortunately make the process simple and easy.
Exercise Caution – Even if you have security software in place, it’s always smart to be wary of suspicious communications and to be on the lookout for unusual behavior on your device. Many top antiviruses can flag suspicious links and phishing messages, but it’s important to think twice before clicking on a link or opening an attachment. For more advice, see our guide to how to avoid phishing scams.

Here are the key details for some of the best providers on the market, based on my broad testing of current security solutions.

Antivirus	Starting Price	Max Devices	Real-Time Protection and Firewall	VPN and Password Manager	Antivirus Lab Testing Scores
TotalAV	$19/year	8	✅ , ❌	✅ , ✅	17/18 (Protection: 5.5/6, Performance: 5.5/6, Usability: 6/6)
Bitdefender	$23.99/year	25	✅ , ✅	✅ , ✅	18/18 (Protection: 6/6, Performance: 6/6, Usability: 6/6)
Norton Antivirus	$29.99/year	Unlimited	✅ , ✅	✅ , ✅	18/18 (Protection: 6/6, Performance: 6/6, Usability: 6/6)
NordVPN Threat Protection	$53.87/year	10	✅ , ❌	✅ , ✅	Excellent testing results
McAfee	$39.99/year	Unlimited	✅ , ✅	✅ , ✅	18/18 (Protection: 6/6, Performance: 6/6, Usability: 6/6)

Conclusion – The Most Destructive Computer Viruses in History

As viruses continue to evolve, it’s important to remain vigilant. Hackers are constantly looking for opportunities to access your system, so it’s essential to follow sound cybersecurity practices – and of course, this list of the most destructive computer viruses is only the tip of the iceberg.

At a minimum, you should ensure you have a reliable antivirus in place and regularly scan your system for threats. You should also keep your operating system and software up to date to ensure you have the latest security patches.

To further protect yourself, look for a multi-layered security system that can encrypt your internet connection with a VPN, protect your login details with a password manager, and provide secure online storage and identity theft protection features.

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