Not to be confused with a virus, a computer worm is a self-replicating program that’s designed to perform some type of malicious activity. They can strike any poorly secured computer, resulting in a fast-spreading infection. Even if you’re familiar with the basic definition of a computer worm, though, there are probably some things you don’t know about them. Below are five facts about computer worms and why they are so destructive.

#1) Spread on Their Own

Computer worms have the ability to spread on their own with the need for a host. This is essentially how they differ from viruses. Most computer viruses work by attaching themselves to a legitimate host program. Once the victim runs the host program, the virus will begin to replicate. Computer worms are more sinister in the sense that they can replicate without the need for a host.

#2) Described in the 1970s

The idea of a self-replicating and self-propagating computer program was originally described in the 1970s by American author John Brunner. During that year, Brunner published a novel titled “The Shockwave Rider” that described a self-replicating malicious program as a “worm.”

#3) A Computer Worm Infected 10% of the Internet

Perhaps the most notorious computer worm of all time is the Morris worm. Developed by Cornell University alumni Robert Morris in the late 1980s, it’s believed to have infected one in 10 internet-connected computers at the time. Thousands of other worms have since emerged, though none have compared to the Morris worm in terms of infectability.

#4) Infect Client Computers and Servers as Well

Computer worms can infect more than just a target computer. Rather, they can also affect client computers and servers on the same network. This is part of the reason why worms have such high infection rates. After infecting a victim’s computer, the worm may spread to other computers and servers on the same network to which it’s connected.

#5) Spread via OS Vulnerabilities

Although they can be spread in many different ways, worms typically make their way onto a victim’s computer via a vulnerability in the computer’s operating system (OS). All computers and servers run at least one OS. Unfortunately, many users neglect to update their device’s OS when a new version is released. The outdated OS may contain a vulnerability that, when exploited, allows a hacker to infect the device with a worm. Keeping the device’s OS up to date is essential for protecting against a myriad of cyber threats, including computer worms.