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.
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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.
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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.
It would be very difficult to even start a free eMail list without mentioning Google's Gmail. Introduced all the way back in 2004 as an invite-only service it became over time the most popular email platform largely due to the fact Google is offering free eMail accounts.
There are a lot of good things to be said about Gmail itself, most of the area is clutter-free and the largest space is reserved for eMail itself letting users focus on what is important. WEB client itself means that you do not need to have any application installed on your device, altho via Google chrome, you can use Gmail offline offering you flexibility if needed.
The ability to connect and manage other accounts like Outlook, Yahoo, etc is just making Gmail even more attractive, and the snooze feature is a really neat little detail that will pause eMail notifications if you need to focus on other things.
Lack of organizing messages into folders is a little confusing since Gmail offers its own unique label system but sometimes I need to have old reliable save eMail to folder.
All in all, Gmail is a great service and it offers a great eMail on the go experience.
Free Windows eMail client simply called Mail is what was once outlook express. Mail itself has the ability to work with other popular accounts like Google Gmail account, Yahoo, iCloud etc. How it comes with Windows OS and it integrates very well with Microsoft Calendar, this eMail client is for many the first choice.
On the negative side, I might say this is a strip-down version of Outlook which is paid solution so some features are missing if we would compare the two.
Overall, a simple and nice eMail client worth your time, especially if you are on the Windows platform.
Great eMail client matching in functions against paid and premium solutions of big tech companies. Fueled with plenty of customization options and reskinning ones this eMail client offers a lot for its free price tag.
It is also fueled by Mozilla community focusing on privacy and security. It can work with any mail service and it is lightweight with a clean look, altho the look itself can be heavily customized.
The bad side is that client itself relies on email services to provide cloud-based emails, so if you are receiving your email via a service that does not have a cloud-based service in itself all of your received emails will be locked to the computer where you have received them. Also customizing it can be sometimes a little too technical for the average computer user.
All in all, Thunderbird is one great eMail client and it would be a shame not to use it because of its technical side, if you need a reliable and secure eMail client on a single machine, look no further than Thunderbird.
Outlook comes as a part of the Microsoft Office suite and as one of the oldest eMail clients, it is still widely popular and adopted through many users and businesses. It has tight integration with all Microsoft services and full integration with Calendar making it one of the if not the best eMail client out there.
Outlook also has a free online service completely free for personal use as well.
The downside is that you can not get it as a separate product if you want a business version other than as a part of the Office suite.
The final verdict would be that this is perhaps the best eMail client out but the big downside is that there is no desktop version outside the Office suite.
eM Client offers a wide array of features, including a calendar, contacts, and chat. Support is provided for all the major email services including Gmail, Yahoo, iCloud, and Outlook.com. The latest version also offers PGP encryption, live backup, basic image editing capabilities, and auto-replies for Gmail.
Its automatic system makes it very easy to get emails from other services since there is no manual setting, all that is needed is to type in your email and eM Client will do everything else automatically.
A one-time purchase is not pricy and it could offer some features that some free clients are missing. Go check it out with a free trial and see if it is for you.
This eMail client's main focus is the simplicity of use with visual appeal while tackling multiple eMail accounts. It has many built-in apps along with a customizable interface. Unlike some more Microsoft-centric email clients, Mailbird Business supports a diverse range of integrated apps, including WhatsApp, Google Docs, Google Calendar, Facebook, Twitter, Dropbox, and Slack, all making for a better-streamlined workflow.
The downside of this client is the yearly subscription plan. I think people, in general, want to get away from software subscription plans so I will include this as a downside but bear in mind it is downside just in terms of a business plan not in the client itself.
Inky is eMail client if you are looking for security. It uses AI along with machine learning in order to block all types of phishing attacks that can get through to other clients. The proprietary machine learning technology can literally read an email to determine if it has phishing content, and then is able to quarantine the email or deliver it with the malicious links disabled. It also takes things a step further and offers an analytics dashboard, which allows an administrator to see patterns of attacks based on dates, or targeted users.
The downside is that client itself is so much focused on security that sometimes some nonsecurity features get overlooked and provide a poor experience but if you need a good and greatly secured eMail client Inky is one to check out. 
