The Most Devastating Computer Viruses in History

The Costliest Computer Viruses

Unless you’re a crypto trader or you have a working design for a time machine, the contents of your computer probably won’t amount to millions, let alone billions, of dollars. But the costliest computer viruses didn’t just target individuals. They targeted businesses, disrupted business operations, stole corporate secrets, and leaked company data. That’s how the following viruses cost their targets billions.

Mydoom

Estimated cost of damage: $38.5 billion

“Mydoom” has such an ominous ring to it, and the virus lived up to its name. In January 2004, Mydoom spread like wildfire through emails. It’s even said that 16 to 25 percent of all emails sent in 2004 were generated by this virus. The sheer amount of spam emails was enough to overload computers and even force businesses to close for days to repair their systems.

The biggest losses came from two companies in particular: The SCO Group and Microsoft. For some reason, the first variant of Mydoom contained a payload scheduled to execute a distributed denial of service (DDoS) attack on The SCO Group’s website on February 1, 2004. The second variant targeted Microsoft’s website two days later. The effect of the attack on Microsoft was minimal, but for The SCO Group, the threat forced it to preemptively shut down its site, costing the company a large amount of its fortune.

Sobig

Estimated cost of damage: $30 billion

At $30 billion, the damage caused by Sobig was — please excuse the pun — so big. Just like Mydoom, Sobig spread through spam emails. Unlike Mydoom, it didn’t have specific targets, and its indiscriminate nature made it all the more destructive.

At the peak of its transmission, which was during the spread of the Sobig.F variant, it clogged and took down the email system at MIT — yes, the Massachusetts Institute of Technology, whose largest and most popular undergraduate program is the Electrical Engineering and Computer Science program.

And that’s the thing about Sobig: It’s not complicated and it doesn’t damage files and computers directly, but the sheer rate at which it spreads and its ability to infect a computer multiple times is enough to clog networks and disrupt processes.

Klez

Estimated cost of damage: $20 billion

Klez is another virus that spreads through email (seeing a pattern here?). Anyway, Klez was destructive because of the manner in which it executes the infection. Like we said, it spreads through emails, but unlike other viruses, one doesn’t need to download the malicious attachment to catch Klez. The virus takes advantage of Outlook and Internet Explorer vulnerabilities to make it so that simply previewing or opening the email executes the virus.

Now, keep in mind, Klez spread in 2001. Microsoft released security updates immediately to patch vulnerabilities, but very few folks updated their systems religiously then. Klez continued to spread to vulnerable computers for months, and even worse, new variants were created, the next one more destructive than the last. Klez continued to spread for the next couple of years, during which time it caused almost $20 billion in damages.

Conficker

Estimated cost of damage: $9 billion

On November 21, 2008, the Conficker worm began its terrible reign. It infected 11 million Windows computers. It spread through various ways including removable drives (think USB flash drives), network shares, self-replication, and weak passwords.

Once the virus contaminated a device it added it to a botnet or group of devices that were infected with malware. Then, those computers were remotely controlled by a hacker. Conficker then began spreading “scareware” which consisted of fake alerts that persuaded victims into downloading malware that they believed was antivirus software.

The Most Widespread Viruses

The costliest computer viruses in history were also some of the most widespread. While the viruses below didn’t cause as much damage, they rivaled Mydoom, Sobig, and Klez in terms of how quickly and vastly they proliferated.

ILOVEYOU

How widespread: Affected 10 million computers

For romantics such as ourselves, catching the love bug is a good thing. But for computer owners in 2000, it wasn’t. The Love Bug virus, otherwise known as the ILOVEYOU virus, infected over 10 percent of the world’s computers. The secret to its success? A self-replicating virus executed with a pinch of social engineering.

The ILOVEYOU virus spread through (surprise, surprise!) emails. The subject line read “ILOVEYOU” with an attached file titled “LOVE-LETTER-FOR-YOU.txt.vbs.” Anyone intrigued enough to find out who sent them the love letter quickly found out that it wasn’t from an admirer. Once a computer was infected, the virus scraped it for email contacts and started spreading itself. And that’s how a simple malicious email infected over 10 million computers worldwide.

Zeus

How widespread: Affected 88 percent of all Fortune 500 companies in 2007

Zeus seized control over computers belonging to 88 percent of all Fortune 500 companies in 2007, and that’s just the start of it. Zeus is a botnet trojan virus. It spread primarily through spam emails (of course!) and drive-by downloads, which is when it piggybacked onto compromised files and executed the infection when someone downloaded the seemingly legitimate file. At one point, it was found in over 76,000 computers worldwide.

Zeus targeted financial institutions in particular, and once a computer was infected, it became a part of a “botnet.” All infected bots gave the “bot master” — a puppeteer of sorts that can simultaneously control all computers in the botnet — backdoor access. And with thousands of computers at its disposal, the bot master could (and did) execute mass attacks. Crimes like DDoS attacks, mass hacking, data breaches, and online banking theft have been linked to the Zeus virus since 2007.

WannaCry/WannaCryptor

How widespread: Affected 200,000 systems, mostly businesses

WannaCryptor, or WannaCry for short, is one of the most devastating and ruthless viruses in recent memory. It’s actually ransomware. Once it infects a computer, it encrypts its files to lock out the owner and then demands a ransom in Bitcoin. If one fails to meet the ransom demands, they’re locked out of their files forever.

WannaCry wasn’t sneaky or clever, but somehow it picked the right targets to cause maximum damage. At one point, Renault, the French car manufacturer, had to close its European manufacturing plants to contain the spread of the virus. Hospitals, airline networks, financial institutions, and even FedEx had fallen victim as well.

Best Computer Virus Prevention Practices

Sharing cybersecurity tips is our bread and butter, and we’re happy to share with you a few things you can do to prevent computer viruses from ever entering your computer.

1. Use antivirus software. This should be obvious. Viruses and malware don’t come knocking on your front door. They hide in email attachments, file downloads, the USB drive of your friend who likes copying movies you’ve downloaded, etc. Your best bet to detect viruses before they infiltrate your computer is antivirus software. For a list of software we recommend, read our guide on the best antivirus software of 2024.
2. Observe safe email practices. We’ve learned that the most destructive viruses primarily spread through emails. So what should we do? Well, first, don’t open emails or download attachments from strangers. They could be malware masquerading as something else. Having some sort of secure email gateway can also help you filter out malicious emails. And, of course, familiarizing yourself with phishing attacks and tactics is very important in preventing social engineering from being used against you.
3. Avoid shady software downloads. That free NBA 2K installer you found in a forum sure seems enticing, but third-party sites offering pirated software are like petri dishes of computer viruses. It’s a best practice not to download anything at all from those sites.
4. Keep your system and programs up to date. If we can learn anything from the most devastating computer viruses in history, it’s that software and firmware updates are important. Most viruses take advantage of security vulnerabilities, but software creators like Microsoft and Apple are often quick to patch those up. If your system is up to date, you’re less likely to fall prey to malware.
5. Keep your firewalls up. Although viruses can enter your computer through USB or external drives, they’re more likely to enter through your network via downloads, the sites you visit, or email attachments. Use your computer’s built-in firewall to filter out malicious traffic, or even better, use antivirus software that offers a firewall itself.