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.
Choosing default browser in settings
Storage management Page update
Another thing with the Steam store management page is the ability to move installation files from one location to another. Let’s say that you have two or more hard disk drivers in your machine and that you have SSD that you use for running stuff since it is fast and larger and slower one for storage. Now you can easily and quickly move one installation from one to another in order to take advantage of your faster SSD for quicker LOAD game times without making a new installation.
Also, you can now drag and drop items in the download bracket to reorder download orders or place them as active downloads to start downloading right away. 
When opened you will be greeted with general settings window. These are settings for power toys themselves, Here you can check for updates, change the looks of power toys, run it on system startup and run them as administrator. Set them up so they best suit your needs.
The color picker will let you as the name suggests pick colors, it will sample colors from running applications and windows, snap their values, and place them in the clipboard. A useful application if you are working as a graphic designer, want to create some cool word document, or just want to compare the difference between colors.
After the Color Picker is activated, hover your mouse cursor over the color you would like to copy and left-click the mouse button to select a color. If you want to see the area around your cursor in more detail, scroll up to zoom in. The copied color will be stored in your clipboard in the format that is configured in the settings (HEX by default). The editor lets you see the history of picked colors (up to 20) and copy their representation in any predefined string format. You can configure what color formats are visible in the editor, along with the order that they appear. This configuration can be found in PowerToys settings. The editor also allows you to fine-tune any picked color or get a new similar color. Editor previews different shades of currently selected color - 2 lighter and 2 darker ones. Clicking on any of those alternative color shades will add the selection to the history of picked colors (appears on the top of the colors history list). Color in the middle represents your currently selected color from the history of the colors. By clicking on it, the fine-tuning configuration control will appear, which will let you change the HUE or RGB values of the current color. Pressing OK will add newly configured color into the history of the colors.
FancyZones is a window manager utility for arranging and snapping windows into efficient layouts to improve the speed of your workflow and restore layouts quickly. FancyZones allows the user to define a set of window locations for a desktop that are drag targets for windows. When the user drags a window into a zone, the window is resized and repositioned to fill that zone. When first launched, the zones editor presents a list of layouts that can be adjusted by how many windows are on the monitor. Choosing a layout shows a preview of that layout on the monitor. The selected layout is applied automatically.
Only 3 options here but maybe some are important to you. This extension lets you enable SVG file preview in File Explorer, enable Markdown preview, and enable SVG thumbnails. Turn on each one you might need.
Image Resizer is a Windows shell extension for bulk image-resizing. After installing PowerToys, right-click on one or more selected image files in File Explorer, and then select Resize pictures from the menu. You can specify your own sizes if you want, you can resize when dragging files, you can overwrite files or make new copies of new sizes, and many more options. A very useful tool I am sure plenty of users can use since this eliminates the need for pictures or any other image application for common resizing tasks.
The PowerToys Keyboard Manager enables you to redefine keys on your keyboard. For example, you can exchange the letter A for the letter D on your keyboard. When you select the A key, a D will display. You can also exchange shortcut key combinations. For example, the shortcut key, Ctrl+C, will copy the text in Microsoft Word. With the PowerToys Keyboard Manager utility, you can exchange that shortcut for ⊞ Win+C). Now, ⊞ Win+C) will copy text. If you do not specify a targeted application in PowerToys Keyboard Manager, the shortcut exchange will be applied globally across Windows. PowerToys Keyboard Manager must be enabled (with PowerToys running in the background) for remapped keys and shortcuts to be applied. If PowerToys is not running, key remapping will no longer be applied.
PowerRename is a bulk renaming tool that enables you to:
PowerToys Run is a quick launcher for power users that contains some additional features without sacrificing performance.
PowerToys Run features include:
This guide uses PowerToys to display common keyboard shortcuts that use the Windows ⊞ key. Windows key keyboard shortcuts can be used while the guide is being shown and the result of those shortcuts (active window moved, arrow shortcut behavior changes, etc) will be displayed in the guide. Releasing the Windows ⊞ key will make the overlay disappear. Tapping the Windows ⊞ key will display the Windows Start menu.
Hey, you made it till the end, thank you for reading and I hope to see you soon. 