What Is the Most Dangerous Computer Virus in History?

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Computer threats are common online, with over a billion circling the web and new viruses created daily.

Most are used to make a mess of your software simply, 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 how it operates.

Once you download a virus, it infiltrates your device disguised as a legitimate and safe program or a document. It 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 specific or all other files and programs, and infecting other devices on the same network.

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

Below are the most prominent computer viruses of all time.

1. Sasser

First on our list of viruses comes Sasser. In 2004, 17-year-old Sven Jaschan created a program that was 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 found a flaw in Windows XP and Windows 2000 and took advantage of a buffer overflow vulnerability in the local security authority subsystem service, which handles security procedures, such as verifying user logins.

The virus would slow down the computer until it crashed, making it hard to reboot the machine without unplugging it from the wall first.

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

2. CryptoLocker

You’ll find ransomware among the different types of viruses lurking on the web. CryptoLocker is a 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.

CryptoLocker stands out because paying the ransom didn’t guarantee the files were decrypted: Many victims reported that the files stayed hidden, lost, or destroyed. CryptoLocker was a big success in the malware world, creating clones like CryptoWall, Crypt0L0cker, and TorrentLocker.

3. Slammer

In 2003, the SQL Slammer worm, 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 again in 2016 from IP addresses in Ukraine, China, and Mexico.

4. CodeRed

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

The virus was lifeless, meaning 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. It 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 viruses.

5. Gameover Zeus

Zeus Trojan is one of the most famous examples of Trojan horse viruses. This virus 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 develop 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 is a botnet network of programs 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% in the US. With 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 criminal organization members behind ZeuS were arrested in an internationally coordinated Operation Tovar.

6. 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 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.

Ultimately, 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. When 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 to become a global pandemic. Within ten 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.

Interestingly, neither Ramones nor de Guzman was convicted or even charged, as there were no malware laws at the time.

8. Klez

Klez was a “mass-mailer” computer virus, listed as a Windows 65-Kb PE EXE file, created within Microsoft Visual C++. It is considered 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. In contrast, the “From” line was generated using the addresses in the Outlook address book. With no obvious message patterns, familiar titles, and “non-generic” subject lines, this computer virus managed to create damage up to $19.8 billion worldwide.

9. Sobig

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

Besides that, Sobig hit the same computer multiple times, meaning 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. 

10. Mydoom

July 26, 2004, was the day Google crashed. It seems strange. But that’s precisely 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, Mydoom sent one in four emails sent globally. But how did it cause Google to crash? The virus flooded its search engine (among others) with automated 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 to email addresses from the user’s address book and even copies itself to any P2P program’s folder, propagating through that network.

It works in two ways:

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

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


Malware has advanced immensely over the years, becoming increasingly sophisticated and destructive. Protecting yourself from these computer threats is crucial today, especially when whole lives seem to be stored online on our clouds, drives, e-wallets, etc. 

Thankfully, antiviruses have evolved, too. If you still need to, we recommend you read our articles on the best antivirus programs for Windows and Mac computers, regularly updated to shield you from even the worst computer virus.

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