5 of The Worst Computer Viruses Ever

Computer viruses, once a laughing matter, are now a crippling and expensive scourge on our internet-connected planet. With an estimated expense of over $55 billion, more than 350,000 new pieces of malware are found every day. But one virus leads the pack with $38 billion in damage—the Mydoom virus in 2004.

The most disruptive computer viruses are ranked by financial impact in this article. But keep in mind that it is just the tip of the iceberg that these malicious programs are. The viruses in this article are just the biggest fish in an endless cybercrime sea, with 127 million new malware apps targeting customers and companies each year.

5 worst viruses in the history of computers

We show the prices, dates, scope, and other main details in the list of the 5 most renowned computer viruses below. Next, a note about terms: we interchangeably use the words “virus” and “worm” since most readers look for them that way. But there is a slight difference that, after the list, we explain.

1. Mydoom

In 2004, Mydoom caused reported damage of $38 billion, the worst computer virus outbreak in history, but its inflation-adjusted cost is now $52.2 billion. This malware, also known as Novarg, is technically a “worm,” transmitted by mass mailing. At one point, for 25 percent of all emails sent, the Mydoom virus was responsible.

Mydoom scraped addresses from infected computers, then sent to those addresses copies of itself. It also roped those infected computers into a computer network called a botnet that attacked distributed denial of service (DDoS). In order to shut down a target website or server, these attacks were intended.

Today, Mydoom is still active, creating 1% of all phishing emails. Considering the 3.4 billion phishing emails sent each day, that’s no small feat. Mydoom has taken on a life of its own by that figure, infecting enough poorly protected computers to send 1.2 billion copies of itself annually, 16 years after its development.

The developer of this dangerous computer worm was never caught, even though a $250,000 reward was offered.

2. Sobig

Currently, another worm is the 2003 Sobig computer virus. In its scale, it is second only to the Mydoom virus. The $30 billion figure, including Canada, the U.K., the U.S., mainland Europe, and Asia, is a worldwide number. In rapid succession, multiple versions of the worm were released, called Sobig.A through Sobig.F, with the most dangerous being Sobig.F.

This cyber-criminal program has masqueraded as legitimate email-attached computer applications. It delayed Air Canada’s ticketing and clashed with numerous other firms. The author of the effective bug has never been caught, despite its widespread damage.

3. Klez

On the list of the worst computer viruses ever made, Klez is a close third. It infected about 7.2 percent of all computers in 2001, or 7 million PCs, with nearly $20 billion in estimated damage. The Klez worm sent fake emails, spoofed known senders, and tried to deactivate other viruses, among other items.

Klez was released in several versions, as with other viruses and worms. It infected files, copied, and distributed itself across the network of each victim. For years, it stuck around, with each iteration being more damaging than the last.
Since most of the computer viruses on this list hit the internet, Windows has come a long way. Thankfully, Microsoft Defender’s built-in security is always on your watch.


The ILOVEYOU virus of the year 2000 operated by sending a fake “love letter” which looked like a harmless text file. This intruder, like Mydoom, sent copies of himself to every email address on the contact list of the infected computer. It spread to more than 10 million PCs shortly after its May 4 publication.

A college student in the Philippines called Onel de Guzman developed the virus. He wrote the virus to steal passwords, lacking money, so he could log into online services he wanted to use free of charge. He had no idea, allegedly, how far his invention would spread. This virus is known as Loveletter, as well.

5. WannaCry

Ransomware, a virus that takes over your machine (or cloud files) and holds them hostage, is the 2017 WannaCry computer virus. In 150 countries, the WannaCry ransomware tore through computers, causing major losses in productivity as corporations, hospitals, and government agencies that didn’t pay were forced from scratch to repair systems.

The malware burned through 200,000 computers worldwide like a wildfire. When a 22-year-old security researcher in the UK stopped, it stopped. A way to turn it off was found. Computers with obsolete operating systems have been particularly hard hit. That’s why security experts still suggest regular upgrading of your systems.

Strikes again

Universal Health Services was struck by one of the potentially biggest computer virus attacks in medical history in September 2020. Damaging ransomware has recently reached the U.S. hospital chain, which has more than 400 locations. The assault caused surgeries to be postponed and made health staff turn to paper records.

Additional notable viruses

Only the ugly tip of a gargantuan digital iceberg are the top 10 worst computer viruses above. With a million new malware programs popping up every 3 years, a few outstanding trees may skip the forest. Here are only a few more viruses that, over the years, have created havoc:

Mimail: In order to unleash a series of DDoS attacks, this worm tried to extract data from infected machines but was relatively easy to delete.
Yaha: Yet another worm with many variants, thought to be the outcome of a Pakistan-India cyber-war.
Swen: Written in C++, to look like a 2003 OS upgrade, the Swen machine worm disguised itself. At $10.4 billion, its financial expense was pegged, Not consistently, though.
Storm Worm: In 2007, this worm showed up and targeted millions of computers with an email about bad weather approaching.
Tanatos/Bugbear: A 2002 keylogger virus that has spread to 150 countries, targeting financial institutions.
Sircam: A 2001 computer worm that used falsified emails with the subject line, “I’m sending you this file to get your advice.”
Explorezip: This worm used fake emails on thousands of local networks to spread to any computer.
Melissa: In 1999, Melissa, the most dangerous computer virus, sent copies of herself that looked like NSFW photos. Cleanup and maintenance costs were estimated by the U.S. FBI at $80 million.
Flashback: a Mac-only virus, infected over 600,000 Macs in 2012 with Flashback and also infected the home base of Apple in Cupertino, Calif. There is now more malware on Macs in 2020 than there is on PCs.
Conficker: This 2009 virus still infects many legacy systems and if it ever activates, it may do serious harm.
Stuxnet: This worm, by sending damaging instructions, is claimed to have damaged Iranian nuclear centrifuges.

Worm vs Virus

The distinction between a virus and a worm is that, like a word processor or web browser, a virus requires another program to make it work. A worm, on the other hand, is self-contained and can run, copy, and submit copies of itself all alone. Some of the computer viruses that are most harmful are simply worms.