We all remember our first computer virus – We knew something was wrong with our computer, but we just couldn’t figure out what.

If you were lucky, you were able to safeguard your data just at the right time, others had to format their entire drives to get rid of the nasty viruses.

Viruses have been around for just as long as computers, and we have still not been able to get rid of them completely. Thousands of new viruses are created every day, and millions of them are circulated around the world. While there are many antivirus software available online, even the best of them cannot detect all the viruses present in a system because new viruses aren’t added to the database in real-time.

Also, since each computer virus has a different motive, not all of them will try to corrupt your system files or try to make your computer go all crazy. Nowadays, many new viruses are aimed at identity theft as they try to steal your banking information. They lay dormant in your system while saving all your passwords and crucial information, and they send it to a different server.

Here are the top 15 computer viruses in the world that you must know about:

15. ILOVEYOU

First seen in May 2000, ILOVEYOU was the first virus to be ever created as a worm. It was a standalone program that was able to replicate itself once it reached a new computer. The virus spread across different users through emails and crashed their computers.

The virus got its name from how it was spread – Users would get an email saying that it was a love letter from their secret admirer. Along with the email, there would be a file attached to the name ‘LOVE-LETTER-FOR-YOU.TXT.vbs. As we can see, the file actually had a .vbs extension, and it was made to seem like a text file. As soon as the user downloaded this file, the virus did not just corrupt his computer, but it would also email itself to first 50 contacts from the user’s address book.

14. Melissa

Melissa was one of the most clever new-age viruses and it was first discovered in March of 1999. It created such havoc among people around the world that it became a breaking news as soon as it spread.

It was spread through email with a subject – ‘Here is that document you asked for, don’t show it to anyone else.’ The email had a file ‘list.doc’ attached, which looked like an ordinary document file but as soon as a user clicked on the file, the virus quickly replicated, checked the user’s address book and mailed itself to the first 50 users with the exact same message and attachment.

13. MyDoom

MyDoom was malware, and unlike other viruses on the list that spread to different users through curious subject lines and messages, this malware chose a different and more standard approach. It spread through an email which looked a lot like a bounced error message with the subject line – ‘Mail Transaction Failed.’ As soon as an unsuspecting user clicked on the email, the attached worm in the email would get executed, spread the worm through the user’s computer and send itself to all the emails in the user’s address book. This denial of service attack was able to infect around 2 million computers.

12. Code Red

Unlike other viruses, Code Red directly attacked Microsoft’s Internet Information Server in 2001, instead of attacking individual users. There were several variations of this virus that went around the internet, but in all the cases, users would get a message saying ‘Hacked by Chinese’ every time they tried to open a web page. The virus was so severe that it was also able to attack White House servers. In less than a week, the virus was able to bring down around 400,000 servers and 1 million PCs. The total estimated damage cost of this virus was $2.6 million.

11. Sasser

Mainly attacking Windows OS, Sasser was a worm virus first seen in 2004. The virus would scan through random IP addresses looking for potential victims. Once it attacked a particular PC, it would scan through the system to check if there were other systems connected to it that could be attacked as well. The virus would contact those systems and instruct them to download the same virus.

The most annoying thing about this virus was that it made it almost impossible to reset the system without cutting the power of the whole system. In a short span of time, the virus was able to attack public transportation, news agencies, and hospitals, and caused an estimated $18 billion in damage.

10. Zeus

A Trojan horse created mainly to infect Windows operated PCs, Zeus was more about performing criminal tasks and less about creating sudden problems for the users.

Once in the system, the Trojan was able to do form grabbing and man-in-the-browser keylogging, and it got into a system through phishing scams. Because of its criminal nature, the Trojan managed to attack many computers and accounts including large banks and corporations like Oracle, Amazon, Bank of America, and Cisco. First noticed in 2009, this virus clearly did a lot of damage to organizations who then had to take strict measures when it came to the security of their servers and computers.

9. Conficker

One of the biggest worm infections that we have ever seen, Conficker first appeared in Windows systems in 2008, and its true author is still unknown. The worm took advantage of Windows’ network service vulnerability which was unpatched in that current Windows version.

After infecting a computer, the worm would block Windows updates and antivirus websites, reset account lockout policies, and turn off other services as well. After that, it would essentially turn the system into a botnet slave and start downloading software to scam money from users. With a damage of around $9 million, the virus was able to affect 9 million computers around the world.

8. Stuxnet

One of the first examples of cyberwarfare, the Stuxnet was created by the American government together with the Israeli Defence Force in an effort to disengage the nuclear programs and efforts of the Iranian government.

The virus was specifically meant to attack Siemens software run in Industrial Programmable Logic Controllers, and it was spread through infected USB drives. If the computer did not contain the Siemens software, then the virus wouldn’t do anything, and it would lay dormant to not get detected.

Stuxnet was able to attack one-fifth of nuclear centrifuges of Iran as 60% of the computers affected by the virus were located in Iran only.

7. CryptoLocker

A Trojan horse ransomware which is still used by hackers around the world, CryptoLocker affects computers running on Windows, and it has several methods through which it can spread itself, but what remains the same is how the Trojan affects the systems.

After a computer is infected, the virus goes on to encrypt important files on the system and any mounted storage. While the virus can be removed from the system, the files stay encrypted, and they can only be recovered by paying a ransom before the deadline specified by the hackers. If the deadline is not met, either the ransom increases, more files are encrypted, or the encryption keys are deleted.

6. Flashback

While Flashback might not be as dangerous as other viruses on the list, it is actually one of the few Mac viruses to ever surface, thus proving that even Macs aren’t completely immune to viruses.

Discovered in 2011, the Flashback was actually a fake Flash extension which would affect Mac computers. In a newer version, this virus was able to attack computers if they were Java enabled (which is likely the case for a majority of computers). The virus would take advantage of JavaScript running on websites and download itself to different computers when the user visited a certain website. Once the virus is in the system, the Mac computer becomes a botnet.

5. Tiny Banker Trojan

Specifically designed to steal banking information, Tiny Banker Trojan is a better and smaller form of Banker Trojan which was seen in the past.

This virus uses packet sniffing to detect if a user is entering login details to operate a bank account. When it does happen, the virus uses bank details like its bank website domain, logo, and title to identify the bank and save its details. After the user has logged in with the correct details, the virus creates a fake pop up with all the bank details to make the page seem real and asks the user for the details again.

4. Shamoon

Discovered by Seculert in August 2012, Shamoon was designed to affect those versions of Microsoft Windows which were based on NT Kernal. If a virus is already in a computer system, then it can move to other systems which are on the same network.

Instead of corrupting, Shamoon would directly start deleting files in the system. It was noticed that the virus was mainly deleting personal files from folders like Downloads, Pictures, Documents, Music, Video, and Desktop. It also had the ability to overwrite the master boot record, making the system unbootable. Shamoon also attacked several energy and oil companies all across the world.

3. Duqu

First found in September 2011, Duqu was quite similar to how Stuxnet worked, but it was completely rewritten to serve a different purpose. The virus got its name from the fact that it made files with a prefix DQ.

It was found that Duqu does not corrupt the system in any way. Instead it lies dormant in a system and only gathers crucial information like document files, desktop screenshots, and keystrokes. The main purpose of Duqu was to steal information, but in some cases, it was found that Duqu was, in fact, deleting some personal files, or the entire hard drive.

The Duqu 2.0 was actually discovered when it was trying to attack the internal system of Kaspersky Labs.

2. Storm Worm

Found in January 2007, Storm Worm is one of the most harmful viruses ever discovered. It started from Russia, and it is the fastest spreading email virus which has ever attacked Windows systems.

Like other viruses which are spread through email, Storm Worm too uses a deceiving headline to make users download email attachments. Usually, the subject made the email look like it was a news piece and the attachment was a video connected to that news.

By June 2007, the Storm Worm had infected over 1.7 million computers, and by September, the number had reached 10 million.

1. Regin

Discovered in November 2014, the Regin virus was so efficient in attacking Windows computers, that most of the security and antivirus software were not able to detect this virus until the very end of 2015.

It is essentially a Trojan horse which gets downloaded to the system when a user visits a spoofed web page. Once downloaded, the virus then downloads more extensions which makes it difficult to be detected by any antivirus.

It has been speculated that this virus was created by the United Kingdom and United States as a way of mass surveillance. But of course, nothing has been proven yet.