11 of the Most Dangerous Computer Viruses and How to Avoid Them

There are more than 450,000 new malware programs found each day, and those are just the ones we know about.[1] You may think computer viruses aren’t something you need to worry about because you’re careful when going online. Many people who end up with costly malware may have thought the same thing.

Computer viruses cost households over $4.5 billion in damages and repair time each year, and some of the most notorious computer viruses have cost more than that in damage over their lifetime.[2] We can’t cover them all, but we’ll highlight some of the worst computer viruses and the damage they inflicted to help you stay informed and alert. We'll also cover how to protect yourself against new computer viruses and the best antivirus programs for the job.

11 most dangerous computer viruses

Learning about the worst computer viruses can show you some of the signs of viruses and how they operate. Here are seven of the nastiest computer viruses in history.

1. Mydoom

Considered by many to be the most dangerous computer virus in history, the Mydoom virus cost around $38 billion worth of damage in 2004. If you think in terms of today’s economy, given inflation, that would amount to about $52 billion.

It worked by scraping email addresses from infected machines. Then it sent copies of the virus to those addresses. The Mydoom virus is still around today and accounts for around 1% of all phishing emails. Devices without solid antivirus protection still get infected and send more than 1 billion copies of the virus each year.

2. Sobig

The Sobig virus is a computer worm. That means it looks for security software vulnerabilities. Before some users knew they got infected by a virus, the worm had spread through their emails.

The Sobig computer virus cost approximately $30 billion in 2003. It affected multiple countries, including the U.S., Canada, and the U.K. The virus was named Sobig because it released several variations of itself, named Sobig.A through Sobig.F, in quick succession. Sobig.F was considered the worst.

3. Conficker

Also known as Downadup, the Conficker virus has infected millions of Windows computers since its creation in 2008. Its most recent version, Conficker C, uses peer-to-peer networking to spread.

Once it contaminated a computer, Conficker added the device to a botnet, or a group of computers infected with malware and remotely controlled by a hacker. Security experts were on edge waiting for Conficker to use its massive botnet to unleash a destructive attack — but it never came. Instead, Conficker was used to spread “scareware,” or fake alerts that try to scare victims into downloading malware disguised as an antivirus program.

4. Klez

The Klez virus cost almost $20 billion in damages. Created in 2001, it infected around 7 million computers.

It worked by creating fake emails pretending to be from legitimate senders. The recipient would open the email and click an attachment only to be infected by the virus. Klez infected computer files, spread throughout the victim’s network, and emailed itself to people in the victim’s address book. Klez was re-released over several years, and each version was more deadly than the previous one.

5. ILOVEYOU

While this virus may have a cute name, it was far from harmless. The ILOVEYOU virus worked like the Mydoom virus, sending copies of itself to every email contact in the infected machine.

In 2000, this was one of the most dangerous computer viruses the world had seen. It sent a text file that appeared harmless and quickly spread to more than 10 million PCs. A college student in the Philippines created the virus to steal passwords to use online services he couldn’t afford for free.

All totaled, the damage amounted to about $15 billion. It was also called the LoveLetter virus.

6. WannaCry

The WannaCry computer virus was one of the first examples of ransomware. It worked by taking over your computer files and then holding them hostage until the victim paid a ransom demand.

Created in 2017, this ransomware virus spread through 150 countries. It infected more than 200,000 computers and cost around $4 billion. A 22-year-old security researcher was able to turn it off. He reportedly could have saved companies billions of dollars by stopping the spread at the time.

The WannaCry virus may have been contained, but it’s still around today. An interesting point is that the creator of the WannaCry virus only demanded a $300 ransom, which means the viral infection was more about disruption than monetary gain.

7. Sasser

Technically a computer worm, Sasser was uncovered in 2004 and targeted computers running unsecure versions of Windows XP and Windows 2000.

Sasser attacked PCs through a vulnerable port, and days after the original worm was released, three new variants appeared: Sasser.B, Sasser.C, and Sasser.D.

Created by a German computer science student named Sven Jaschan, Sasser dealt damage worldwide. The worm caused the University of Missouri to disconnect from the internet, Delta Air Lines canceled multiple international flights, and the British Coast Guard had to disable its electronic mapping system.

8. Zeus

Zeus is considered one of the most dangerous computer viruses because it breached about 88% of all Fortune 500 companies. It also cost $3 billion in total damages and was responsible for about 44% of all banking malware attacks in 2007, the year it was created.

The virus was a group of programs that took over machines remotely. It sent out phishing emails and used malicious downloads to infect new hosts. Once the user clicked on the attachments or links, the virus installed the Zeus malware package.

The Zeus virus is particularly nasty because it can steal your sensitive data, such as banking information. (It’s also known as a banking Trojan horse.) The other way it works is by remotely contacting the hacker so they can hijack your computer and install more malware.

9. Code Red

The Code Red worm was discovered in 2001. The virus infected 975,000 computers and displayed the words, “Hacked by Chinese” on affected web pages. The Code Red virus left no trace of its existence in hard drives or other computer storage.

Code Red’s total damages amounted to about $2.4 billion. Before it finished spreading, it even attacked the White House’s website, forcing it to change the IP address to defend against the virus.

10. Cryptolocker

Yet another notorious virus that targets the Windows operating system, Cryptolocker is a Trojan horse that encrypts files on your device and any connected media. Once it strikes, victims see a countdown timer and a warning that their files have been encrypted.

Although many antivirus programs were able to remove the Trojan, they couldn’t decrypt data. This forced several victims to reinstall Cryptolocker in order to pay the ransom for their data. This highlights the importance of maintaining a digital backup of any critical files you may have on your computer.

11. Stuxnet

You may remember hearing about the computer worm Stuxnet around 2010. It gained notoriety for infecting Windows machines through USB drives and attacking Iran’s nuclear facilities. Some call Stuxnet the first cyberweapon.

Along with interfering with Iranian nuclear centrifuges, Stuxnet also released a rootkit to cause further destruction. If businesses didn’t have strong cybersecurity programs in place before, they likely scrambled to get up to speed after this infamous computer worm was discovered.

How to protect against new computer viruses

Reading about the devastation the most dangerous computer viruses have caused can show you how easy it is to become vulnerable to an attack. Thankfully, you can take the following steps to prevent a hacker or virus from penetrating your device:

1. Use antivirus software

Knowing how antivirus software works to protect your online security is crucial. When you install antivirus on your device, you give yourself an added layer of protection against malware.

Your antivirus software will scan programs for any suspicious files. You can choose to delete the files or quarantine them until you decide what to do with them.

Some of our favorite antivirus programs that offer free and paid plans include:

• Avast: We think Avast offers some of the best free antivirus scanning and protection, which is great if you are on a budget or if you want to try the service first. If you opt for a paid plan, Avast Premium Security offers Targeted Scans, Smarts Scans, and Deep Scans, which we found easy to use.
  Get Avast | Read Our Avast Review
  
• AVG: If you're looking for optimal online and device security, AVG is the antivirus for you. Tools like Wi-Fi verification, a phishing site blocker, and ad tracking help protect against commonly missed risks.
  Get AVG | Read Our AVG Review 
• Bitdefender: Bitdefender provides a full suite of security features at a budget-friendly price. On top of malware, ransomware, and spyware protection, Bitdefender also comes with identity theft protection in case you do ever become the target of a phishing attack. 
  Get Bitdefender | Read Our Bitdefender Review

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2. Update your software and operating system

Many software programs and operating systems show you a notification when an update is available. Those updates may obtain fixes for bugs that could leave you vulnerable to a computer virus.

Your antivirus software needs to stay updated to catch the latest computer viruses. Because new malware gets created every day, your software can’t catch what it doesn’t recognize. Similarly, updates to your operating system can patch new security vulnerabilities that hackers may try to exploit.

A simple update can mean the difference between a computer virus infecting your device and your data remaining secure.

3. Avoid anything that looks suspicious

You may think that seems simple, but keep in mind how the most dangerous computer viruses we’ve mentioned were spread. These may have seemed like harmless attachments before the users opened them. Once opened, they were able to infect the device, and in some cases, spread through an entire network of computers.

If something looks odd or suspicious, don’t open it. Use caution when visiting websites and don’t click on pop-up ads. These often contain malware.

FAQs about computer viruses

Bottom line

Seeing how the most dangerous computer viruses can sneak in and gain access to so many devices gives you more reason to use caution when online.

The unfortunate reality is there are still hackers who want to get into your most sensitive data, either for fun or financial gain. That doesn’t have to deter you from working or having fun online.

The right tools can help you stay safe and keep your devices secure. When you use strong antivirus software and avoid suspicious emails, you’re more likely to prevent cybercriminals from hacking into your device.

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Author Details

Patti Croft

About the Author

Patti Croft is a seasoned writer specializing in technology, with three years of experience. With a B.S. in Computer Science and a background as a technical analyst and security specialist, she covers a range of topics like data security and parental control software.