What Is the Most Dangerous Computer Virus in History?

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Computer threats are a common occurrence in the online world, with over a billion of them circling the web and more being created every day. 

Most are used to simply make a mess of your software, others to gather information about the computer or the user, and some to blackmail you for money. 

Not all strains of malware are created equal. Some can cause minor annoyances, while some have cost companies billions of dollars, invaded government IT systems, and even shut down Google. They’re responsible for the worst computer virus attacks in history.

What Is a Computer Virus?

A computer virus is software designed to infiltrate and take control of your device, altering the way it operates. 

Once you download a virus, it infiltrates your device disguised as a legitimate and safe program or a document and can lie dormant until you run the specific program it was attached to. 

From that moment on, the virus runs through your computer doing whatever it was created to do, which may include gathering information, restricting your access to certain or all other files and programs, and infecting other devices on the same network.

Most viruses are easy to shield from – there are hundreds of excellent antivirus tools to pick from that’ll protect your computer from most of the malware crawling around.

10. Sasser

First on our list of viruses, comes Sasser. In 2004, 17-year-old Sven Jaschan created a program that was actually supposed to help people get rid of the most dangerous computer virus ever – MyDoom – but instead became responsible for one of the worst computer virus attacks in history. 

Sasser was able to find a flaw in the Windows XP and Windows 2000 and take advantage of a buffer overflow vulnerability in the local security authority subsystem service, whose function is to handle security procedures, such as verifying user logins. 

The virus would slow down the computer until the point of crashing and make it hard to reboot the machine without unplugging it from the wall first.

It infected millions of computers, but what made it special is that it didn’t require the user to click on a specific file to activate it – Sasser was able to spread across the network without any human interaction at all and it did so worldwide, dealing damage to government agencies, airlines, and many other organizations, making an estimated damage cost of $500 million.

9. CryptoLocker

Among the different types of viruses lurking on the web, you’ll find ransomware. CryptoLocker is one such piece of malicious code. It would attack by encrypting files and then displaying a red ransom note on the screen, followed by a payment window note. 

It’s estimated that the damage cost was around $665 million as it hit over 5,000 companies, some of which decided to pay the ransom. 

What makes CryptoLocker stand out is that paying the ransom didn’t guarantee the files being decrypted: Many victims reported that the files stayed hidden, lost, or destroyed. CryptoLocker was a big success in the malware world, leading to the creation of clones like CryptoWall, Crypt0L0cker, and TorrentLocker. 

8. Slammer

In 2003, the SQL Slammer worm, also called Sapphire, selected random IP addresses, exploited vulnerabilities, and spread across numerous computers. Once on the computers, it would launch a distributed denial-of-service attack on several internet hosts, significantly slowing down internet traffic. 

It hit ATMs in the US and Canada, a 911 emergency response system in Washington State, and even a nuclear plant in Ohio.

All in all, this computer virus caused damage estimated at $1.2 billion. It emerged once more in 2016, coming from IP addresses in Ukraine, China, and Mexico.

7. CodeRed

Named CodeRed due to the two eEye Digital Security employees drinking CodeRed Mountain Dew at the moment they discovered it in 2001, this piece of malware seemed relatively innocuous compared to some other computer viruses from our list until it found and exploited a flaw in Microsoft Internet Information Server. 

The virus was “fileless,” meaning that you didn’t have to click on anything to activate it, it just needed a stable internet connection. It left close to no trace in the computer’s memory and used the infected devices to target websites with distributed denial-of-service attacks, displaying the now-famous message “Hacked by the Chinese.”

The most famous example was the takedown of the White House’s website, making it change its IP address to defend against the breach. Many other government institutions were forced to take down their websites as well. The financial cost came at $2.4 billion, earning it the #7 spot on our list of the most dangerous of all viruses.

6. Gameover ZeuS

One of the most famous examples of Trojan horse viruses, ZeuS is used to create new viruses, swipe passwords and files, and help create a genuine underground market for compromised identities that can be purchased for as little as 50 cents. It’s the same Trojan horse used to create and replicate the CryptoLocker ransomware. 

ZeuS can bypass centralized servers, creating independent ones to send sensitive information and prevent the victim from even tracing their stolen data. It functions as a botnet network of programs working together to transfer funds to secret accounts. 

It’s estimated that ZeuS is behind 44% of all bank malware attacks, breaching 88% of companies from the Fortune 500 list. ZeuS infected over a million computers worldwide, with 25% being in the US. With both documented and undocumented damage in mind, loss of productivity, and removal, it’s estimated that this computer virus cost the world around $3.7 billion.

In 2010, over 100 members of the criminal organization behind ZeuS were arrested in an internationally coordinated Operation Tovar. 

5. WannaCry

You’ll hardly find a more suitable name for a virus, especially if it’s ransomware. WannaCry emerged in 2017, encrypting files and demanding a 0.1 BTC ransom transferred to the hackers’ Bitcoin address. It affected 200,000 computers in 150 countries, most of them running on outdated Windows systems.

Microsoft quickly released a patch to resolve the issue, but not before the virus hit the UK’s National Health Service, causing almost $120 million in damage, as it infected 70,000 of its devices, not sparing even MRI scanners and theater equipment.

In the end, the total cost of productivity loss, stolen and lost assets, and decryption was pegged at $4 billion. The NHS faced heavy criticism in the aftermath of the breach, as Windows XP, which it was using at the time, was 17 years old. 


Also known as LoveLetter, ILOVEYOU was considered to be the most dangerous virus at the time of its release. The ILOVEYOU virus was created by two Philippine programmers, Reonel Ramones and Onel de Guzman, to steal passwords they could use to log in to online services they wanted to use for free. 

The virus contained a TXT document nicknamed something similar to Love-Letter-For-You.TXT.vbs or FRIEND_MESSAGE.TXT. The moment an unsuspecting victim clicked to open the file, it would mail itself to the first 50 email addresses from the PC’s address book.

It only took hours for it to become a global pandemic and within 10 days, it’s said to have infected over 45 million devices including Pentagon, CIA, and the UK Parliament computers.

With 10% of the world’s computers infected, the overall damage cost of this virus is estimated to be over $10 billion.

It’s interesting that neither Ramones nor de Guzman were convicted or even charged as there were simply no laws about malware at the time.

3. Klez

Klez was a “mass-mailer” computer virus, listed as a Windows 65-Kb PE EXE file, created within Microsoft Visual C++ and considered to be one of the worst viruses ever due to its high stealth technique most common antivirus software tools can’t detect. 

It accessed the computer through an infected email. The subject line was randomly selected from a list of possible choices while the “From” line was generated using the addresses located in the Outlook address book. With no obvious message patterns, familiar titles, and “nongeneric” subject lines, this computer virus managed to create damage up to $19.8 billion worldwide.

2. Sobig

Sobig was another email spammer that appeared just two years after Klez. It had multiple versions, starting with Sobig.A all the way up to Sobig.F, but with a twist – it had its own built-in email software, unlike previous viruses that had to rely on programs like Microsoft Outlook and rarely spread to rival email software. 

Apart from that, Sobig was able to hit the same computer multiple times, which means that the number of infections can’t be directly compared to other viruses.

Sobig didn’t physically damage computers, files, or any critical data, but forced networks to shut down outside access to its email system, by tying up computer and networking resources.

It’s estimated that Sobig created a productivity loss of $30 billion worldwide. 

1. Mydoom

July 26, 2004, was the day Google crashed. Seems strange, doesn’t it? But that’s exactly what happened. 

Considered to be the most destructive virus in history, the Mydoom virus was the only one to spread even faster than ILOVEYOU. Its record still stands – at its peak, one in four emails sent globally was sent by Mydoom. But how did it cause Google to crash? The virus flooded its search engine (among others) with automatic search queries as it tried to find valid email addresses. 

Mydoom spreads itself by mimicking a failed email transmission that contains a file of its own. Once executed, it sends itself to email addresses from the user’s address book and even copies itself to any P2P program’s folder, propagating itself through that network.

It works in two ways:

  • Opens up a backdoor to the infected computer, allowing the hacker remote control
  • Launches a distributed denial-of-service attack aimed at one of the most hated companies in tech – SCO. (It is, in fact, believed that Mydoom was created due to conflict with SCO over ownership of Linux code.)

The damage Mydoom caused was at the time estimated to be $38 billion dollars, which today would be a bit over $57 billion. And it’s still circulating today, 16 years after it was launched.


Through the years, malware has advanced immensely, becoming more and more sophisticated and destructive. Protecting yourself from these computer threats is crucial in today’s world, especially when whole lives seem to be stored online on our clouds, drives, e-wallets, etc. 

Thankfully, antiviruses have evolved, too. If you haven’t yet, we recommend you read our articles on best antivirus programs both for Windows and Mac computers, which are regularly updated to shield you from even the worst computer virus.

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