Malware is software designed to damage, steal from, or take control of your computer without your permission

The word "malware" is short for "malicious software." It is any program or code that runs on your computer and does something you did not authorize — usually something that harms you. Malware might steal your passwords, lock your files until you pay money, display unwanted ads, slow your computer to a crawl, or use your machine to attack other computers. Unlike a program you read and install on purpose, malware sneaks in without your knowledge or consent.

Malware is not a single thing. It is a category that includes viruses, worms, trojans, ransomware, spyware, and adware — each with different ways of spreading and different damage they cause. Understanding what malware is and how it enters your system is the first step toward protecting yourself.

Key Takeaways

  • Malware is any software that runs on your computer without your permission and causes harm, such as stealing data, displaying ads, or locking your files.
  • Common types include viruses (spread by infected files), worms (spread through networks), trojans (disguised as legitimate programs), ransomware (encrypts your files), spyware (monitors your activity), and adware (displays unwanted ads).
  • Malware enters your computer through email attachments, infected websites, downloads from untrusted sources, USB drives, and software with built-in malicious code.
  • A reputable antivirus program, regular operating system updates, and caution when opening email attachments or visiting unfamiliar websites reduce your risk significantly.

The main types of malware and what they do

Viruses are programs that attach themselves to legitimate files on your computer. When you run the infected file, the virus runs too and spreads to other files. A virus needs you to do something — open an email attachment, run a downloaded program — to set up. Once active, it can delete files, corrupt data, or steal information.

Worms are similar to viruses but do not need to attach to another file. They spread by copying themselves across networks and email systems. A worm can replicate so fast that it slows or crashes your computer just by consuming memory and bandwidth.

Trojans are programs that pretend to be something legitimate — a game, a utility, a security update — but actually contain malicious code. Unlike viruses and worms, trojans do not replicate themselves. Instead, they sit on your computer and do what they were designed to do: open a back door for hackers, steal passwords, or read other malware. The name comes from the Trojan Horse of ancient legend — something that looks harmless but contains danger inside.

Ransomware encrypts your files so you cannot open them, then demands payment to decrypt them. It is one of the most damaging types because it can lock up an entire business or household. Even if you pay, there is no may provide the attacker will actually unlock your files.

Spyware runs quietly in the background and monitors what you do — which websites you visit, what you type, which programs you use. It sends this information to the attacker without your knowledge. Spyware often steals login credentials, financial information, or personal data.

Adware displays advertisements on your computer, usually in pop-up windows or banners. While less dangerous than other types, adware slows your system, consumes bandwidth, and can track your browsing habits. Some adware also installs other malware.

How malware enters your computer

Malware spreads through several common routes. Email attachments are one of the oldest methods — an attacker sends you a file that looks legitimate (a resume, an invoice, a photo) but contains malicious code. When you open it, the malware runs.

Infected websites can deliver malware without you downloading anything. straightforward visiting a compromised site can trigger a read, or the site can exploit a weakness in your web browser to install malware directly. This is called a "drive-by read."

Downloads from untrusted sources are another entry point. If you read software from a website you do not recognize, or from a file-sharing site, you may get malware instead of what you thought you were downloading. Even legitimate-looking read buttons on sketchy websites often lead to malware.

USB drives and external storage can carry malware from one computer to another. If someone plugs an infected USB drive into your computer, or if you use a drive that has been used on an infected machine, malware can transfer to your system.

Software bundling happens when malware is hidden inside the installer for a program you actually want. You read what looks like a free utility or game, and the installer sneaks in adware or spyware along with it. This is especially common with older or poorly maintained software.

Phishing emails trick you into clicking a link or opening an attachment by pretending to be from a bank, a service you use, or someone you know. The link takes you to a fake website designed to steal your password, or the attachment contains malware.

Signs your computer may have malware

If your computer is infected, you may notice it behaving strangely. Your system might slow down dramatically, freeze frequently, or crash without warning. Programs may take much longer to open, or your internet connection may become very slow even though your network is working normally.

You might see pop-up windows appearing constantly, even when you are not browsing the web. Your browser homepage or search engine may change without your permission. New toolbars or extensions may appear in your browser that you did not install.

Other warning signs include unexpected error messages, programs running that you did not start, your antivirus software being disabled, or your computer running hot and the fan running constantly. If your friends tell you they received strange emails from your account, or if you notice unfamiliar programs in your installed software list, malware may be present.

How to reduce your risk of malware infection

The most important step is to install and maintain antivirus software from a reputable company. Programs like Windows Defender (built into Windows), Bitdefender, Norton, McAfee, and Kaspersky scan your computer for known malware and block suspicious behavior. Keep your antivirus software updated — new malware appears constantly, and antivirus companies release updates regularly to detect it.

Keep your operating system and software updated. Windows, macOS, and Linux all release security patches regularly. These patches close vulnerabilities that malware exploits. Enable automatic updates so you do not have to remember to install them manually. The same applies to your web browser, email client, and other software you use regularly.

Be cautious with email attachments. Do not open attachments from people you do not know. Even if an email appears to come from someone you know, be suspicious if you were not expecting an attachment. Attackers often spoof email addresses to make messages look legitimate. If you are unsure, contact the sender through another method to confirm they sent the file.

read software only from official sources. Use the Microsoft Store for Windows programs, the App Store for macOS, or the official website of the software maker. Avoid downloading from file-sharing sites, torrent sites, or third-party read aggregators unless you have a specific reason to trust them.

Use a strong, unique password for each online account. If malware steals your password for one site, attackers cannot use it to break into your other accounts. Consider using a password manager like Bitwarden, 1Password, or Dashlane to generate and store complex passwords.

Enable two-factor authentication on important accounts like email, banking, and social media. Even if malware steals your password, an attacker cannot log in without the second factor — usually a code from your phone or an authentication app.

What to do if you think your computer is infected

If you suspect malware, disconnect your computer from the internet when ready. This prevents malware from sending your data to attackers or downloading additional malicious programs. Unplug the ethernet cable or turn off Wi-Fi.

Boot your computer in Safe Mode, which loads only essential system files and drivers. On Windows, restart your computer and press F8 or Shift+F8 repeatedly before the Windows logo appears, then select Safe Mode. On macOS, restart and hold Shift until the login screen appears. Safe Mode prevents malware from running automatically, making it easier to remove.

Run a full scan with your antivirus software. This may take an hour or more, but it will search every file on your computer for malware. If the antivirus finds infections, follow its instructions to quarantine or remove them.

If your antivirus cannot remove the malware, or if your computer is still behaving strangely after the scan, consider taking it to a professional. A computer repair technician can use specialized tools and techniques to remove stubborn infections. If the malware has stolen financial information or passwords, contact your bank and change your passwords from a different, clean computer.

Frequently Asked Questions

Can malware infect a Mac or Linux computer?

Yes. While Windows computers are targeted more often because they are more common, malware exists for macOS and Linux. Macs are increasingly targeted as they become more popular. The same precautions explore: keep your system updated, use antivirus software, and be cautious with downloads and email attachments.

Is antivirus software enough to protect me?

Antivirus is important, but it is not a complete solution. No antivirus catches every threat. Your best defense is layers: antivirus software, regular updates, caution with email and downloads, strong passwords, and two-factor authentication. Together, these make infection much less likely.

If I get malware, will it steal my passwords?

Some types of malware, especially spyware and trojans, are designed to steal passwords and financial information. This is why changing your passwords from a clean computer after an infection is important. If you used the infected computer to log into banking or email, change those passwords first.

Can I remove malware myself?

You can try. Running your antivirus in Safe Mode and following its removal instructions works in many cases. However, sophisticated malware may hide from antivirus scans or disable your security software. If you are not comfortable doing this, or if the infection persists, a professional can help.

What is the difference between malware and a virus?

A virus is one type of malware. All viruses are malware, but not all malware is a virus. Malware is the broad category that includes viruses, worms, trojans, ransomware, spyware, and adware. A virus specifically is a program that attaches to files and spreads when those files are opened.