What Are The Most Destructive Computer Viruses Worldwide



What is a computer virus?

A computer virus is a malicious piece of computer code designed to spread from device to device. A subset of malware, these self-copying threats are usually designed to damage a device or steal data.

Think of a biological virus – the kind that makes you sick. It’s persistently nasty, keeps you from functioning normally, and often requires something powerful to get rid of it. A computer virus is very similar. Designed to replicate relentlessly, computer viruses infect your programs and files, alter the way your computer operates or stop it from working altogether.

A computer virus is a malicious piece of computer code designed to spread from device to device. A subset of malware, these self-copying threats are usually designed to damage a device or steal data.

Think of a biological virus – the kind that makes you sick. It’s persistently nasty, keeps you from functioning normally, and often requires something powerful to get rid of it. A computer virus is very similar. Designed to replicate relentlessly, computer viruses infect your programs and files, alter the way your computer operates or stop it from working altogether.

Examples of computer viruses

In 2013, the botnet  virus Gameover ZueS was discovered to use peer-to-peer downloading sites to distribute ransomware and commit banking fraud. While tens of thousands of computer viruses still roam the internet, they have diversified their methods and are now joined by several malware variants like:

  • Worms – A worm is a type of virus that, unlike traditional viruses, usually does not require the action of a user to spread from device to device.
  • Trojans – As in the myth, a Trojan is a virus that hides within a legitimate-seeming program to spread itself across networks or devices.
  • Ransomware – Ransomware is a type of malware that encrypts a user’s files and demands a ransom for its return. Ransomware can be, but isn’t necessarily, spread through computer viruses.

What does a computer virus do?

Some computer viruses are programmed to harm your computer by damaging programs, deleting files, or reformatting the hard drive. Others simply replicate themselves or flood a network with traffic, making it impossible to perform any internet activity. Even less harmful computer viruses can significantly disrupt your system’s performance, sapping computer memory and causing frequent computer crashes.

What are the symptoms of a computer virus?

Your computer may be infected if you recognize any of these malware

  • Slow computer performance
  • Erratic computer behavior
  • Unexplained data loss
  • Frequent computer crashes


6 worst computer viruses

Cybercrime is a growth industry. The cost of cybercrime will continue to increase as more businesses go online around the world and connect to the Internet. In fact, the components of malicious cyber activity do not only entail cybercrime but also the loss of sensitive business information, employment disruptions, reduced trust for online activities, and reputational damage. When putting these together along with the cost of cyber espionage, the global cybercrime cost is measured in the tens of trillions of dollars. Cyber security ventures predicts that global cybercrime costs will hike up to over 10 trillion USD annually by the year 2025. Now have a look at the 6 worst computer viruses in history.


Klez was written in Microsoft Visual C++ and was one of the most disastrous computer viruses in history. Due to its highly stealth and anti-anti-virus technic, the most common anti-virus software cannot detect Klez. Klez is known to be a “mass-mailer”, sending itself to randomly selected addresses from the user’s address book in Outlook. Once the virus grabs an email address in the infected machine, it turns it into the “from” address. Klez arrives in e-mails with different subject lines. Since there was not any familiar title or message pattern, the virus has, unfortunately, became successful.


Sasser was first discovered in April of 2004. This virus targeted computers that run on vulnerable versions of Microsoft Windows XP and Windows 2000. It was written by a German computer science student named Sven Jaschan. Do you know what makes Sasses one of the worst computer viruses?

Sasser can spread without any human intervention, unlike most viruses that require a user’s click. It propogates through a Windows vulnerability known as Local Security Authority Subsystem Service. big organizations were unlikely to be impacted by the Sasser virus as their firewalls would easily keep the virus out, whereas small and medium-size businesses were most at risk from Sasser. Do you wonder who has been hit? Great Britain’s Maritime and Coastguard Agency and Taiwan’s postal service were hit by Sasser.


ILOVEYOU (a.k.a. Love Bug) is one of the worst computer viruses of all time. It was 2000, Windows users started to receive emails with the subject line “I love you”. Within about 10 days, 45 million infections were reported. It has been believed that at least 10% of the internet-connected computers in the world impacted by this virus.

Forbes says that the Pentagon, the CIA, and the U.K. Parliament all shut down their email systems for several hours in response to the incident. In fact, ILOVEYOU is one of the first worms to successfully utilise social engineering to prompt victims to open attachments. The malicious code was designed to steal passwords and automatically deliver copies of itself to all contacts in the victim’s Microsoft Outlook address book.

Shamoon was discovered in August of 2012 that targeted 32 bit NT kernel versions of Microsoft Windows. This modular computer virus was known to be created for cyber warfare and it was capable of blocking the infected computer from booting.

Shamoon has been used in some of the most devastating cyberattacks of all times, for example, the 2018 attack on an Italian oil and gas enterprise called Saipem. The outcome was really damaging, that it caused over 300 computers to stop operating and affected its servers in the Middle East, said Reuters In 2012, Shamoon infected the infrastructure of two Middle East-based energy companies named Saudi Aramco and RasGas Company Limited.


Nimda first showed itself in September of 2001 and it was a mass-mailing virus that utilised several tactics to spread itself. The name of the virus stemmed from the reversed spelling of “admin”. Nimda spreads via infected email attachments and across websites running vulnerable versions of Microsoft’s IIS web server software. Big corporations like including Microsoft and Dell were among the victims of Nimda.

One of the biggest lessons to be learned from Nimda is how critical it is that organisations patch known holes inside their networks  quickly. According to kaspersky, there are several variants of the Nimda virus. Most of them are known to be a patched version of the original virus where the text strings are replaced with other strings.

1) Stuxnet (2009-2010) The arrival of Stuxnet was like a cartoon villain come to life: it was the first computer virus designed specifically to cause damage in the real, as opposed to virtual, world. While previous malware programs may have caused secondary physical problems, Stuxnet was unique in that it targeted software that controls industrial systems. Specifically, Stuxnet was designed to damage machinery at Iran’s uranium enrichment facility in Natanz. Based on the available information, including data from the International Atomic Energy Agency, experts believe Stuxnet caused a large number of Iran’s centrifuges—essentially giant washing machines used to enrich uranium—to spin out of control and self-destruct. Though Stuxnet was discovered in 2010, it is believed to have first infected computers in Iran in 2009.

2) Conficker Virus (2009)In 2009, a new computer worm crawled its way into millions of Windows-based PCs around the world, creating a massive botnet army of remotely controlled computers capable of stealing financial data and other information. Its complexity made it difficult to stop, and the virus prompted the creation of a coalition of experts dedicated to stopping its spread. At its height, the Conficker worm infected millions of computers, leading anti-virus researchers to call it the “super bug,” or “super worm.” But the real mystery of Conficker, which still infects a large number of computers, is that no one knows what it was meant to do: the botnet army was never used for any specific purpose, to the best of anyone’s knowledge. Conficker’s real purpose still confounds security experts.

3) agent.btz (2008) This piece of malware’s claim to fame is that it temporarily forced the Pentagon to issue a blanket ban on thumb drives and even contributed to the creation of an entirely new military department, U.S. Cyber Command. Agent.btz spreads through infected thumb drives, installing malware that steals data. When agent.btz was found on Pentagon computers in 2008, officials suspected the work of foreign spies. Former Deputy Secretary of Defense William Lynne later wrote that agent.btz created “a digital beachhead, from which data could be transferred to servers under foreign control.” Though some anti-virus experts have disputed the contention that the virus was the creation of a foreign intelligence agency, its effect was to make cyber war a formal part of U.S. military strategy.

4) Zeus (2007) There is no shortage of malware kits that target personal information, but Zeus has become the go-to tool for many of today’s cyber criminals and is readily available for sale in the cyber crime underworld. It can be used to pilfer passwords as well as files, helping to create a literal underground economy for compromised identities that can be bought and sold for as little 50 cents. In the age of Internet banking and online shopping, a compromised identity is much more than just a name and social security number: it’s your address, date of birth, mother’s maiden name, and even your secret security questions (your first pet, your favorite teacher, or your best friend from grade school).

5) PoisonIvy (2005) PoisonIvy is a computer security nightmare; it allows the attacker to secretly control the infected user’s computer. Malware like PoisonIvy is known as a “remote access trojan,” because it provides full control to the perpetrator through a backdoor. Once the virus is installed, the perpetrator can activate the controls of the targeted computer to record or manipulate its content or even use the computer’s speaker and webcam to record audio and video. Once thought of as a tool for amateur hackers, PoisonIvy has been used in sophisticated attacks against dozens of Western firms, including those involved in defense and chemical industries, according a white paper written by Symantec, the computer security firm. The attacks were traced back to China.

6) MyDoom (2004) MyDoom muscled its way into the malware world in 2004, quickly infecting some one million computers and launching a massive distributed denial of service attack, which overwhelms a target by flooding it with information from multiple systems. The virus spread through email as what appeared to be a bounced message. When the unsuspecting victim opened the email, the malicious code downloaded itself and then pilfered the new victim’s Outlook address book. From there, it spread to the victim’s friends, family and colleagues. MyDoom spread faster than any worm seen prior.

7) Fizzer (2003) By 2003, many worms were spreading over e-mail, but Fizzer was an entirely new creature. If earlier worms, like Code Red (see below), were about mischief, Fizzer was all about money. While some initially dismissed the seriousness of the worm because it wasn’t as fast moving as Code Red, Fizzer was more insidious. “What makes Fizzer stand out is that it’s the first instance of a worm created for financial gain,” says Roel Schouwenberg, a senior researcher at Kaspersky, an anti-virus company. “Computers infected with Fizzer started sending out pharmacy spam.” In other words, Fizzer didn’t just take over your address book to spread for the sake of spreading, it used your address book to send out the now familiar porn and pills spam. Fizzer was followed by better-known spam-inducing worms, like SoBig, which became threatening enough that Microsoft even offered a $250,000 bounty for information leading to the arrest of its creator.

8) Slammer (2003) In January 2003, the fast-spreading Slammer proved that an Internet worm could disrupt private and public services, a harbinger for future mayhem. Slammer works by releasing a deluge of network packets, units of data transmitted over the Internet, bringing the Internet on many servers to a near screeching halt. Through a classic denial of service attack, Slammer had a quite real effect on key services. Among its list of victims: Bank of America’s ATMs, a 911 emergency response system in Washington State, and perhaps most disturbingly,  a nuclear plant in Ohio

9) Code Red (2001) Compared to modern malware, Code Red seems like an almost kinder, gentler version of a threat. But when it swept across computers worldwide in 2001, it caught security experts off guard by exploiting a flaw in Microsoft Internet Information Server. That allowed the worm to deface and take down some websites. Perhaps most memorably, Code Red successfully brought down the whitehouse.gov website and forced other government agencies to temporarily take down their own public websites as well. Though later worms have since overshadowed Code Red, it’s still remembered by anti-virus experts as a turning point for malware because of its rapid spread.


Computer virus protection

When you arm yourself with information and resources, you’re wiser about computer security threats and less vulnerable to threat tactics. Take these steps to safeguard your PC with the best computer virus protection:  

  • Use antivirus protection and a firewall
  • Get antispyware software
  • Always keep your antivirus protection and antispyware software up-to-date
  • Update your operating system regularly
  • Increase your browser security settings
  • Avoid questionable Websites
  • Only download software from sites you trust.
  • Carefully evaluate free software and file-sharing applications before downloading them.  
  • Don’t open messages from unknown senders
  • Immediately delete messages you suspect to be spam

An unprotected computer is like an open door for computer viruses. Firewalls monitor Internet traffic in and out of your computer and hide your PC from online scammers looking for easy targets. provide complete protection from the two most dangerous threats on the Internet – spyware and computer viruses. They prevent viruses from entering your computer, stand guard at every possible entrance of your computer and fend off any computer virus that tries to open, even the most damaging and devious strains.  

While free antivirus downloads are available, they just can’t offer the computer virus help you need to keep up with the continuous onslaught of new strains. Previously undetected forms of polymorphic malware can often do the most damage, so it’s critical to have up-to-the-minute, guaranteed antivirus protection.

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