Computers viruses, worms, ransomware, etc. are kinds of malicious software that no user should take lightly. On several occasions, we have touched upon security steps that each user should take in order to protect its identity and data. Sadly sometimes even when all precautions are taken some malware can still slip through and wreak havoc. Today we are looking at some of the worst or the best, depending on your view that has indeed wreaked plenty of havoc.
10 worst computer viruses in history
In the list of the 10 most famous computer viruses below, we show the costs, dates, reach, and other key facts. First a note about terms: we use the words “virus” and “worm” interchangeably because most readers search for them that way. But there’s a subtle difference that we explain after the list.
1. Mydoom – $38 billion
The worst computer virus outbreak in history, Mydoom caused estimated damage of $38 billion in 2004, but its inflation-adjusted cost is actually $52.2 billion. Also known as Novarg, this malware is technically a “worm,” spread by mass emailing. At one point, the Mydoom virus was responsible for 25% of all emails sent.
Mydoom scraped addresses from infected machines, then sent copies of itself to those addresses. It also roped those infected machines into a web of computers called a botnet that performed distributed denial of service (DDoS) attacks. These attacks were intended to shut down a target website or server.
Mydoom is still around today, generating 1% of all phishing emails. That’s no small feat considering the 3.4 billion phishing emails sent each day. By that figure, Mydoom has taken on a life of its own, infecting enough poorly-protected machines to send 1.2 billion copies of itself per year, 16 years after its creation.
Though a $250,000 reward was offered, the developer of this dangerous computer worm was never caught.
Wondering what makes the world’s most secure computers so safe? See the Tech@Work guide: Upgrade to the World's Most Secure and Manageable PC
2. Sobig – $30 billion
The 2003 Sobig computer virus is actually another worm. It is second only to the Mydoom virus in its scope. The $30 billion figure is a worldwide total, including Canada, the U.K., the U.S., mainland Europe, and Asia. Several versions of the worm were released in quick succession, named Sobig.A through Sobig.F, with Sobig.F being the most damaging.
This cybercriminal program masqueraded as legitimate computer software attached to emails. It disrupted ticketing at Air Canada and interfered with countless other businesses. Despite its widespread damage, the creator of the successful bug was never caught.
3. Klez – $19.8 billion
Klez is a close third on the list of the worst computer viruses ever created. With nearly $20 billion in estimated damages, it infected about 7.2% of all computers in 2001, or 7 million PCs. The Klez worm sent fake emails, spoofed recognized senders and, among other things, attempted to deactivate other viruses.
As with other viruses and worms, Klez was released in several variants. It infected files, copied itself, and spread throughout each victim’s network. It hung around for years, with each version more destructive than the last.
Windows has come a long way since most of the computer viruses on this list hit the web. Thankfully, built-in protection with Microsoft Defender is always on the watch.
4. ILOVEYOU – $15 billion
The year 2000’s ILOVEYOU virus worked by sending a bogus “love letter” that looked like a harmless text file. Like Mydoom, this attacker sent copies of itself to every email address in the infected machine’s contact list. Shortly after its May 4 release, it had spread to more than 10 million PCs.
The virus was created by a college student in the Philippines named Onel de Guzman. Lacking funds, he wrote the virus to steal passwords so he could log into online services he wanted to use for free. He reportedly had no idea how far his creation would spread. This virus is also known as Loveletter.
Need to up your remote work security game before there’s another entry on the list of most deadly computer viruses? See our guide: How to Work Remotely and Securely
5. WannaCry – $4 billion
The 2017 WannaCry computer virus is ransomware, a virus that takes over your computer (or cloud files) and holds them hostage. The WannaCry ransomware ripped through computers in 150 countries, causing massive productivity losses as businesses, hospitals, and government organizations that didn’t pay were forced to rebuild systems from scratch.
The malware raged like wildfire through 200,000 computers worldwide. It stopped when a 22-year-old security researcher in the U.K. found a way to turn it off. Computers with out-of-date operating systems were hit especially hard. That’s why security experts always recommend updating your systems frequently.
Ransomware strikes again
In September 2020, one of the potentially largest computer virus attacks in medical history hit Universal Health Services. The U.S. hospital chain, which has more than 400 locations, was reportedly struck by damaging ransomware. The attack forced the cancellation of surgeries and made healthcare workers switch to paper records.
6. Zeus – $3 billion
The Zeus computer virus is an online theft tool that hit the web in 2007. A whitepaper by Unisys three years later estimated that it was behind 44% of all banking malware attacks. By then, it had breached 88% of all Fortune 500 companies, 2,500 organizations total, and 76,000 computers in 196 countries.
The Zeus botnet was a group of programs that worked together to take over machines for a remote “bot master.” It originated in Eastern Europe and was used to transfer money to secret bank accounts. More than 100 members of the crime ring behind the virus, mostly in the U.S., were arrested in 2010. It’s not as prominent today, but some of the virus’ source code lives on in newer botnet viruses and worms.
Zeus caused documented damage of $100 million. But the real cost in terms of lost productivity, removal, and undocumented theft is undoubtedly much higher. A $3 billion estimate, adjusted for inflation, puts this virus at a cost of $3.7 billion in today’s dollars.
7. Code Red – $2.4 billion
First observed in 2001, the Code Red computer virus was yet another worm that penetrated 975,000 hosts. It displayed the words “Hacked by Chinese!” across infected web pages, and it ran entirely in each machine’s memory. In most cases it left no trace in hard drives or other storage.
Financial costs are pegged at $2.4 billion. The virus attacked websites of infected computers and delivered a distributed denial of service (DDoS) attack on the U.S. White House’s website, www.whitehouse.gov. In fact, the White House had to change its IP address to defend against Cod Red.
Can your printer get a virus? See our cool infographic: The State of Printer Security
8. Slammer – $1.2 billion
The SQL Slammer worm cost an estimated $750 million across 200,000 computer users in 2003. This computer virus randomly selected IP addresses, exploiting vulnerabilities and sending itself on to other machines. It used these victim machines to launch a DDoS attack on several internet hosts, significantly slowing internet traffic.
The Slammer worm hit banks in the U.S. and Canada especially hard, taking ATMs offline in many locations. Customers of Toronto’s Imperial Bank of Commerce found themselves unable to access funds. The attack reared its ugly head again in 2016, launching from IP addresses in Ukraine, China, and Mexico.
9. CryptoLocker – $665 million
Thankfully, ransomware attacks like the 2013 CryptoLocker virus have dipped since their 2017 peak. This malware attacked upwards of 250,000 machines by encrypting their files. It displayed a red ransom note informing users that “your important files encryption produced on this computer.” A payment window accompanied the note.
The virus’ creators used a worm called the Gameover Zeus botnet to make and send copies of the CryptoLocker virus. According to a report by security firm Sophos, the average ransomware attack costs a business $133,000. If we estimate that CryptoLocker hit 5,000 companies, that would put its total cost at $665 million.
Where will cybersecurity go next? See our guide: The Future of Cybersecurity
10. Sasser – $500 million
The Sasser worm was written by a 17-year-old German computer science student named Sven Jaschan. He was arrested at the age of 18 in 2004 after a $250,000 bounty was posted for the computer virus’ creator. A friend of Jaschan’s tipped authorities that the youth had penned not only the Sasser worm but also the damaging Netsky.AC attack.
Jaschan was given a suspended sentence after it was found he was a minor when he wrote the malware. The Sasser worm crashed millions of PCs, and though some reports put damages at $18 billion, the relatively low infection rate suggests a more likely cost of $500 million.
Other notable viruses
The top 10 worst computer viruses above are just the ugly tip of a gargantuan digital iceberg. With a million new malware programs popping up every 3 years, we may miss the forest for a few outstanding trees. Here are just a few more viruses that have wreaked havoc over the years:
Mimail: This worm tried to harvest data from infected machines to launch a string of DDoS attacks, but was relatively easy to remove.
Yaha: Yet another worm with several variants, thought to be the result of a cyber-war between Pakistan and India.
Swen: Written in C++, the Swen computer worm disguised itself to look like a 2003 OS update. Its financial cost has been pegged at $10.4 billion, but not reliably.
Storm Worm: This worm showed up in 2007 and attacked millions of computers with an email about approaching bad weather.
Tanatos/Bugbear: A 2002 keylogger virus that targeted financial institutions and spread to 150 countries.
Sircam: A computer worm from 2001 that used counterfeit emails with the subject line, “I send you this file in order to have your advice.”
Explorezip: This worm used fake emails to spread to every machine on thousands of local networks.
Melissa: The most dangerous computer virus in 1999, Melissa sent copies of itself that looked like NSFW pics. The U.S. FBI estimated cleanup and repair costs at $80 million.
Flashback: A Mac-only virus, Flashback infected over 600,000 Macs in 2012 and even infected Apple’s home base in Cupertino, Calif. In 2020, there’s now more malware on Macs than on PCs.
Conficker: This 2009 virus still infects many legacy systems and could do significant damage if it ever activates.
Stuxnet: This worm is reported to have destroyed Iranian nuclear centrifuges by sending damaging instructions.
⊞ WINDOWS + N - notification panel
⊞ WINDOWS + W - News and Interests feed
With the News and Interests feed, Windows 11 offers users the ability to check the latest news, the weather, and more without opening a browser window.
⊞ WINDOWS + Z - Snap Layout
Instead of the usual two windows snapping capability, Windows 11 allow users to snap their windows in a three-column layout. Users can access this by pressing Windows Key and Z to access the Snap layout.
⊞ WINDOWS + PRT SCN - taking screenshots
This one isn’t exactly new, but pressing the Windows Key and Print Screen will automatically capture the entire screen and save a copy of the image on your PC. Once the screen flashes, you will know that the screenshot has been taken, after which you can find the file saved under your Pictures folder in a subfolder called Screenshot.
⊞ WINDOWS + C - Microsoft Teams chat
Microsoft Teams app, the service allows anyone with a Microsoft account to quickly start a chat using the Windows + C shortcut. 
Coursera was one of the pioneers when it comes to college education online. When it started all courses were free and offered certificates if the taker has done tasks with a passing grade. Today not everything is free but still, there are tons of free courses that can help you to learn something new and to advance in your work or hobby.
Google is using the Coursera platform to deliver its courses to the audience and besides google a lot of academic professors from Princeton, Stanford, John Hopkins and many more are presently offering their course material.
edX has evolved from MITx, MIT’s free initiative to place some free academic lectures from its classrooms free into the world. Since it was rebranded into edX it started including other big university names like Berkeley, University of Texas System, Harvard, and many more.
Since its infant days, edX has expanded and included many areas that it offers coursework from arts to science. Certificated today are mostly not free but course videos are, so if you care to learn and not wave certificates around this is a great place to do it.
Udemy has a little different approach to course material than previously mentioned websites. Teachers on Udemy are mostly people that are working in a given field. Programmers will teach programming, fitness instructors will teach training, etc.
There are vas variety of topics even on lifestyle, hobbies even gaming to more serious ones like software developing hardware, etc.
There are free courses but mostly are one-time purchases with unlimited access to course material and with frequent discounts udemy might be a good place to look if you can find anything interesting.
Alison is kind of Udemy if we are talking about course material and who is teaching, the difference however is that it has more free content and it is one of the rare websites on this list that has Language courses.
Courses vary from beginner level to more advanced ones.
Udacity was also one of the earlier websites to offer free courses, in today’s world it still has free material but there is also paying one.
The big difference from other competitors is that Udacity focuses mostly on programming, data science, and web development and it is very strong in this field. There are no courses about art. Music or similar, mostly IT stuff is covered here.
If this is something that interests you, make a free account and give it a go.
Codecademy is a website mainly focused on teaching you how to code. Pro plan is there but there are a lot of basic and free courses on the mentioned subject.
The curriculum is really wide covering Python, R, Java, JavaScript, SQL, Ruby, C#, C++, Swift, PHP, HTML, CSS and more so I am sure that if you are interested in coding this would be a pretty good place to find things.
Besides pure language coding courses site offers also, Cybersecurity, Web design, Data science, web development, Game Development, Mobile Development, and many more courses on provided subjects.
Academic Earth has been built with one simple premise: Everyone deserves access to education.
Lead by this kind of mindset on Academic Earth you will find well, academic courses similar to ones found on edX and Coursera, but from mentioned sites here all courses are free and you will find only academic ones, there are no entrepreneur or expert courses here, just pure academic ones.
If you wish to see and learn something the way it is taught in Harvard, Berkeley, MIT, etc this is the place for you.
Khan academy is completely free and it is offering courses matching certain grades for children. It is offering courses for all 8 years of elementary school, math courses for high school and college with some coursework on topics like science, arts & humanities, reading, life skills, etc.
Despite not being on the same level as some university courses presented on other mentioned websites Khan Academy is a good place to learn a lot of basic skills that you could build upon later.
