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The late 1990s were a particularly scary time for computer viruses. Most users were still learning basic cybersecurity habits. As a result, they—and the machines they used—were vulnerable to viruses that spread in what we would now consider obvious ways. That was the true power of the Melissa virus.

Melissa was initially uploaded to pornographic websites, but it soon spread to millions of Windows machines via email throughout 1999. Once opened, the infected email triggered the virus to forward itself to the first 50 contacts in the user’s Outlook address book. The virus rapidly infiltrated personal, corporate, and even government networks. It took a coalition of agencies to slow the infection, clean infected systems, and eventually arrest its creator. It caused an estimated $80 million in damages and served as a chilling preview of the digital threats to come.

Imagine it’s 2001 and you receive an email from someone you trust. The message seems a little odd but harmless. Even if you’re aware of early cybersecurity risks, you might still open it. That’s exactly how the Klez virus managed to infect over 7% of computers worldwide.

Klez’s real power came from its ability to spoof the identities of trusted contacts. Even cautious users fell victim. The virus’s functionality varied depending on which version you received, but its distribution method made it especially hard to stop. It forced a generation of users to reconsider whether any email could be trusted.

Like many early viruses, the 2003 SoBig worm spread via email, often through vague yet seemingly innocuous messages. Unlike many of its predecessors, however, SoBig had self-updating capabilities that enabled it to adapt and survive evolving defenses.

The SoBig variants remain some of the fastest-spreading worms in history. Believed to have caused over $30 billion in damages, the virus forced major public and private networks to temporarily shut down. It was only stopped when it deactivated itself. To this day, its creator has never been caught or definitively identified.

Code Red earned its name from the flavor of Mountain Dew being consumed by the researchers who discovered it in 2001. But there was nothing sweet about what the worm actually did.

Code Red infamously displayed the message “HELLO! Welcome to [website name] Hacked By Chinese!” on infected systems. More concerning was its behind-the-scenes activity: launching distributed denial-of-service (DDoS) attacks on major IP addresses, including the White House, which had to change its IP to mitigate the damage. The worm caused billions in financial losses and highlighted dangerous vulnerabilities in supposedly secure networks.

Cryptolocker was an early and terrifying example of ransomware, a type of virus that encrypts your data and demands a ransom to unlock it. First detected in 2013, it wasn’t the first of its kind, but it was one of the most influential.

Cryptolocker typically spread via email attachments. Once installed, it encrypted the user’s files and demanded a Bitcoin payment to restore access. Its use of sophisticated cryptography made manual recovery nearly impossible. Before a broader solution was found, it infected hundreds of thousands of systems and ushered in a new era of cybercrime-for-profit.

CIH, also known as “Chernobyl,” was created in the late 1990s by a Taiwanese student who claimed he wanted to challenge top antivirus developers. Whether it started as a prank or protest, it quickly became one of the most destructive viruses ever made.

In some instances, CIH attacked and overwrote a computer’s BIOS, rendering the machine completely unbootable. Though rare, such attacks essentially “killed” the computer. The virus prompted a major reevaluation of hardware security protocols. The student behind CIH eventually went on to develop antivirus software as a form of atonement.

While many early viruses were loud and disruptive, Zeus took a more insidious approach. First discovered in 2007, this virus’s goal was to silently steal money.

Zeus infiltrated computers through malicious links and emails, then quietly recorded keystrokes and harvested login credentials. Its targets included Fortune 500 companies, banks, and everyday users. It remained active for nearly a decade and inspired numerous spinoffs. Unlike flashier viruses, Zeus demonstrated just how quietly a cyberthief could operate—and how costly that silence could be.

Speed is critical when it comes to computer viruses—and no worm spread faster than MyDoom. Released in 2004, it remains the fastest-propagating email worm in history.

MyDoom disguised itself in innocent-looking emails. Once activated, it initiated a DDoS attack while opening backdoors into the infected system. Its ability to replicate via contact lists made it nearly impossible to contain. At its peak, MyDoom accounted for up to 30 percent of global email traffic and caused over $35 billion in damages.

It may sound like a joke, but ILOVEYOU—a worm sent with the subject line “I Love You”—was anything but funny. Unleashed in 2000, this unsophisticated virus wreaked havoc through sheer volume.

All it took was one person clicking on a seemingly affectionate message. The virus would then overwrite files, harvest the user’s address book, and forward itself to everyone on the list. The damage was swift and staggering, infecting tens of millions of systems. Its legacy endures as a reminder of how easily trust can be manipulated.

You probably never encountered Stuxnet—and that was by design. Widely believed to be a joint U.S.-Israeli cyberweapon, this virus targeted Iran’s nuclear program and marked a chilling new chapter in cyberwarfare.

Stuxnet infiltrated industrial systems involved in uranium enrichment and altered the behavior of physical machinery while displaying false normal readings. It didn’t just crash systems; it sabotaged a nation’s infrastructure from within. Stuxnet proved that malware could cross the boundary from the digital world into real-world consequences—and opened the door to a future of militarized code.