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What Is the Most Dangerous Computer Virus?

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A computer virus is a malicious code or program that alters your computer operating system. A computer virus works by attaching or inserting itself into a legitimate document or program on your computer that supports macros to execute its code. However, all computer viruses aren’t the same, and some are more dangerous than others. So, what is the most dangerous computer virus? 

Quick Answer

There are thousands of computer viruses, but one of the most dangerous of all times is Mydoom. Other worst computer viruses in history include Sobig, Klez, ILOVEYOU, WannaCry, etc. 

A virus can be on your computer without you even knowing. Also, some viruses can stay dormant with no signs or symptoms; some will infiltrate your computer and steal sensitive information like your password by logging keystrokes, corrupt files, spam your email contacts, etc.

There was a time when viruses were a thing that cost many people and organizations billions because antivirus wasn’t as advanced as they are now. 

Continue reading this article to learn about some of the worst computer viruses in history in this article. 

Some of the Worst Computer Viruses of All Time 

There are several types of computer viruses. Some of the most common computer viruses are web scripting, browser hijacker, overwrite, resident, etc. These viruses can get into your computer through your email, web browser, USB, etc. 

While you can protect your computer from a virus or malware attack by using an antivirus or avoiding suspicious websites or links, viruses sometimes find their way to your computer despite being very careful. 

Below are some of the worst computer viruses of all time. 

Virus #1: Mydoom 

Mydoom, also known as Novarg, is technically a malware worm that is spread by mass emailing. Mydoom is one of the worst viruses in history, as it caused estimated damage of $38 billion in 2004. And if we were to adjust for inflation, that should amount to approximately $52.2 billion today.

When Mydoom gets on a device, it infects it and then sends copies of itself to other email addresses on the computer and infects those computers. In other words, the Mydoom virus infected the computer into a web of computers called a botnet which performs DDoS attacks intended to shut down a target server or website. 

Virus #2: Sobig 

Another well-known computer virus is the Sobig of 2003. This computer virus is also a worm-like malware that costs over $30 billion in damage worldwide, including in Asia, Europe, the US, the UK, and Canada. There were several versions of the virus in circulation, such as the Sobig.A through to Sobig.F, where Sobig.F was the worst

When a computer is infected with Sobig, it camouflages itself as legitimate computer software by attaching itself to emails. Its spread was so severe that it disrupted ticketing at Air Canada and several other businesses. And despite its widespread damage, authorities never caught its creator. 

Virus #3: Klez 

Klez is another computer virus considered the third worst globally. It cost nearly $20 billion in damage and infected over 7.2% of computers in 2001. The Klez malware also posed as a fake email, infecting the computer, copying itself, and spreading to other PCs.  

Like other worms and viruses, Klez was also released in several variants. This malware hung around for years, with each new version being more destructive than the last. Windows has come a long way since then, and the built-in Microsoft Defender can protect against viruses like this. 

Virus #4: ILOVEYOU 

Another well-known virus is the ILOVEYOU virus which became popular in the 2000s and cost over $15 billion in damage. However, like Mydoom, this file attacks the PC, copies itself, and sends itself to other email addresses in the infected computer contact list. 

Shortly after its release in May, it spread to over 10 million PCs. This virus was created by a Philippine college student Onel de Guzman, who wrote the code to steal passwords for online services he wanted to use for free. 

Virus #5: WannaCry 

WannaCry is a ransomware computer virus that made its rounds on the internet in 2017. When this virus infects a computer, it takes over and holds it hostage. This ransomware affected over 150 countries leading to losses in productivity, affecting hospitals, government organizations, businesses, etc. 

This virus led to a $4 billion loss for organizations, forcing them to rebuild their system. This malware ranged like wildfire until a security researcher from the UK found a way to protect computers from it. 

Virus #6: Zeus 

Zeus is another computer virus that hit the web in 2007. It is an online theft tool that leads to over 44% of all bank malware attacks. About 88% of all Fortune 500 companies and 2500 organizations in around 196 countries were affected by this virus. 

The Zeus virus originated from Eastern Europe and was used to move money to secret bank accounts. The Zeus attack led to over $100 million of documented damage. But the actual cost of damage due to reduction in productivity, undocumented theft, etc., is undoubtedly over the $3 billion estimate

Virus #7: Code Red 

Code Red is another computer virus that first came to light in 2001. It is a worm-like malware that infiltrated almost 1 million hosts. When a computer is infected with this virus, the infected page will have the words “Hacked by Chinese” displayed. 

When the Code Red virus infects a computer, it leaves no trace in the hard drive or other storage. This virus was mainly targeted at websites and delivered as a DDoS attack. On average, this virus causes a lot of financial loss, which is pegged at around $2.4 billion

Virus #8: Slammer 

Slammer is a computer virus that surfaced in 2003 and was able to spread over 200,000 computers. It uses its victims’ computers to launch a DDoS attack, significantly slowing internet traffic. The SQL Slammer randomly selects IP addresses, exploits their vulnerability, and sends itself to other computers that connect to that computer. 

The Slammer virus hits the Canadian and US banks, taking their ATM offline in several locations. In Toronto’s Imperial Bank of Commerce, they couldn’t access funds. The Slammer attack resurfaced in 2016 and was launched from IP addresses in Mexico, China, and Ukraine. Overall, this virus led to over $1.2 billion in damage

Keep in Mind

While viruses and worms damage your computer, they are pretty different. A worm is self-contained and runs, copies, and sends copies of itself to other computers, while a virus needs another program to work, such as a web browser.


As you can see, computer viruses are dangerous and can cause serious problems. It can make you lose your files, equipment, or even money. Although you can easily detect if there is a virus on your PC today due to technological advancement, things weren’t as easy back in the day.

If you have an antivirus on your PC, a file, or a website that possesses any security threat, the antivirus will warn you immediately. However, if you are unfortunate to get a virus on your PC, you can get rid of it by scanning your PC with antivirus and removing corrupted files. 

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