An internet worm is a program that copies itself across computers without you running it

A worm is a piece of software that spreads from one computer to another on its own, without needing you to open a file or click a link. Unlike a virus, which attaches itself to a program you have to run, a worm finds its own way in through a gap in your system — usually an unpatched security flaw or a weak password — and then makes copies of itself on other machines.

The worm does not need your help to move. It scans the network (your home network, your workplace, or the internet at large) looking for other computers with the same security gap, breaks in, and copies itself there. Then it does the same thing from that new computer. This is why worms spread so fast — they work automatically, 24 hours a day, while you are using your computer normally.

The damage a worm causes depends on what it was built to do. Some worms just copy themselves endlessly until they slow your computer down or fill up your hard drive. Others steal passwords, delete files, or turn your computer into a tool that sends spam or attacks other machines without your knowledge.

Key Takeaways

  • A worm spreads on its own by finding security gaps in your operating system or network, not by waiting for you to open something.
  • Worms move faster than viruses because they do not need human action — they automatically copy themselves from machine to machine.
  • The most common entry point is an unpatched security flaw, which is why keeping your operating system and software updated matters.
  • A worm can turn your computer into a tool for attacking other machines or sending spam without you realizing it is happening.

How a worm finds its way into your computer

A worm looks for a specific weakness — a security flaw that the software maker has not yet fixed, or one that you have not installed the fix for yet. When it finds a computer with that weakness, it exploits it to get inside. This is different from a virus, which usually waits for you to read an infected file or open an email attachment.

The most common entry points are outdated software. If your operating system (Windows, macOS, Linux) has a known security hole and you have not installed the patch that closes it, a worm can slip through. The same is true for older versions of web browsers, email programs, or any software that connects to the internet. A worm does not care whether you are paying attention — it just needs the gap to exist.

Weak passwords are another route in. If a worm has already infected one computer on your network and it tries to access others using common passwords like "password" or "123456", it may succeed. Once it is in, it copies itself and repeats the process on the next machine.

The difference between a worm and a virus

People often use the words "worm" and "virus" interchangeably, but they work differently. A virus attaches itself to a file — a document, a photo, an executable program — and spreads only when you open that file. A virus needs your action to move. A worm does not.

Think of it this way: a virus is like a disease that spreads when you shake someone's hand. A worm is like a disease that spreads through the air on its own. Both are harmful, but the worm does the work of spreading itself.

Because worms do not need you to do anything, they tend to spread much faster and wider than viruses. A major worm outbreak can infect millions of computers in hours. A virus outbreak usually takes longer because it depends on people opening files.

What happens when a worm infects your computer

When a worm gets in, you might not notice anything at first. It may run quietly in the background, copying itself and spreading to other machines. But over time, you will likely see signs. Your computer may slow down dramatically because the worm is using your processor and internet connection to spread. Your hard drive may fill up with copies of itself. Your internet bill might spike if the worm is sending data out without your knowledge.

Some worms are designed to cause obvious damage right away. They might delete files, corrupt your operating system, or display messages on your screen. Others are sneaky. They might turn your computer into a bot — a machine controlled remotely by the person who wrote the worm — and use it to send spam, steal data, or attack other computers. You would not know it was happening.

In a workplace or school network, a worm can spread to hundreds of machines in minutes. This is why IT departments take worm outbreaks seriously and sometimes shut down networks to stop the spread.

How to protect yourself from worms

The most important defense is keeping your software up to date. When a software maker discovers a security flaw, they release a patch — a small update that closes the gap. If you install patches as soon as they are available, you remove the entry points that worms look for. Most operating systems can be set to install updates automatically, and you should turn that on.

Use a strong password on every account, especially on your computer itself and your router. A strong password has at least 12 characters and mixes uppercase letters, lowercase letters, numbers, and symbols. Avoid words from the dictionary or personal information like birthdays. A worm that tries common passwords will fail if yours is genuinely random.

Run antivirus or antimalware software and keep it updated. These programs scan your computer for known worms and remove them. They also watch for suspicious behavior — a program trying to copy itself across your network, for example — and alert you. Windows comes with Windows Defender built in, and it works well if you keep it turned on.

On your home network, use a router with a firewall enabled. A firewall blocks incoming connections that you did not ask for, which stops many worms from entering in the first place. Most home routers have this turned on by default, but check your router settings to be sure.

Famous worms and what they did

The Morris Worm, released in 1988, was one of the first major worms. It spread through the early internet and infected about 10 percent of all computers connected at the time. It did not destroy data, but it slowed systems down so much that many universities and research centers had to shut down their networks. The person who wrote it, Robert Morris, was prosecuted and became the first person convicted under the Computer Fraud and Abuse Act.

The ILOVEYOU worm appeared in 2000 and spread through email. It arrived as an attachment with the subject line "ILOVEYOU" and, when opened, deleted files and sent copies of itself to everyone in the victim's address book. It infected millions of computers worldwide in a few days and caused billions of dollars in damage.

The Conficker worm emerged in 2008 and exploited a flaw in Windows. It infected millions of computers, including many in government agencies and large companies. It was designed to steal data and turn infected computers into bots, but security researchers managed to limit its spread by blocking the servers it tried to contact.

What to do if you think your computer has a worm

If your computer is running slowly, your hard drive is filling up fast, or you see unfamiliar programs running, you may have a worm. The first step is to disconnect from the internet — unplug your ethernet cable or turn off Wi-Fi — so the worm cannot spread to other machines or receive commands from whoever controls it.

Then run a full scan with your antivirus software. Make sure the software is up to date before you scan. If the scan finds something, let it remove it. If your computer is badly infected and will not start properly, you may need to restart it in Safe Mode (a limited version of your operating system that loads only essential programs) and scan from there.

If the worm has damaged your operating system and your computer will not work even after removal, you may need to reinstall Windows, macOS, or Linux from scratch. This is a last resort, but it guarantees the worm is gone. Back up any important files to an external drive first, if you can do so safely.

Frequently Asked Questions

Can a worm infect my phone or tablet?

Yes, but it is less common. Phones and tablets run different operating systems (iOS, Android) with different security models. Android phones are more vulnerable than iPhones because Android is more open. Keep your phone's operating system updated and read apps only from official stores like Google Play or the Apple App Store.

Will antivirus software stop a worm before it spreads?

Good antivirus software can catch many worms, but not all. If a worm is brand new and the antivirus company has not added it to their database yet, the software may not recognize it. This is why keeping your operating system patched is more important than relying on antivirus alone — you are closing the door before the worm can knock.

If I have a worm, can it see my passwords and bank information?

It depends on what the worm was designed to do. Some worms include keylogging software that records everything you type, including passwords. Others steal data from your hard drive. If you suspect a worm has been on your computer, change your passwords from a different device and monitor your bank accounts for unauthorized activity.

Can I get a worm just by visiting a website?

A worm cannot infect you just by visiting a normal website. However, if a website has been hacked and is hosting malicious code, and your browser has an unpatched security flaw, a worm could potentially get in. This is rare, but it is another reason to keep your browser updated.

Do Mac computers get worms?

Yes, though less frequently than Windows computers. Worms target whatever is most common and most vulnerable. Windows has the largest market share, so most worms are written for Windows. But macOS and Linux can be infected too, especially if they are not kept up to date.