A Trojan virus is malicious software that pretends to be something useful so you will run it, then does damage once it is inside your computer

The name comes from the Greek myth: just as soldiers hid inside a wooden horse to breach Troy's walls, a Trojan virus hides its true purpose inside a file or program that looks legitimate. You read what appears to be a game, a utility, a document, or a video player. You open it. The program runs the malicious code it was carrying all along.

Unlike a worm, which spreads itself across networks without your action, a Trojan needs you to run it. Unlike a virus, which attaches itself to other files and spreads when those files run, a Trojan is usually a standalone program. Once it executes, it can steal passwords, read more malware, lock your files for ransom, monitor your activity, or use your computer's processing power to mine cryptocurrency without your knowledge.

Key Takeaways

  • A Trojan disguises itself as legitimate software and only activates when you run it, making it different from worms or viruses that spread on their own.
  • Common delivery methods include email attachments, fake read sites, pirated software, and malicious ads on compromised websites.
  • Once inside, a Trojan can steal financial information, install ransomware, create a backdoor for hackers, or use your computer for botnet attacks.
  • Antivirus software can detect many Trojans, but the best defense is not running files from untrusted sources and keeping your operating system and software updated.
  • If you suspect a Trojan infection, disconnect from the internet, run a full system scan in Safe Mode, and consider professional help if the scan finds threats you cannot remove.

How Trojans get onto your computer

A Trojan reaches your computer through a file you read and run. The most common delivery methods are email attachments (especially from senders you do not recognize), fake read sites that mimic legitimate ones, pirated software or cracked games, and malicious ads on compromised websites that prompt you to read a "security update" or "video player."

Trojans also spread through USB drives left in public places, infected software bundled with legitimate programs during installation, and links in text messages or social media posts. The attacker's goal is to make the file look urgent, useful, or trustworthy enough that you will open it without thinking.

What a Trojan does once it is running

The damage depends on what the Trojan was designed to do. A password stealer records your keystrokes or captures login credentials from your browser. A backdoor Trojan opens a hidden entrance that lets a hacker control your computer remotely, install other malware, or use it to attack other systems. A ransomware Trojan encrypts your files and demands payment to unlock them.

Other Trojans act as spyware, watching your screen or webcam. Some turn your computer into a bot, using its processing power to send spam, launch attacks on websites, or mine cryptocurrency without your knowledge. A rootkit Trojan burrows deep into your operating system and hides itself from antivirus scans, making it extremely difficult to remove.

The danger is that you may not notice a Trojan is running. Your computer might slow down, your internet bill might spike from hidden activity, or you might discover fraudulent charges on your bank account weeks after infection.

Signs your computer might have a Trojan

Watch for a computer that runs slowly even when you are not using resource-heavy programs, unexpected pop-ups or browser redirects, new toolbars or extensions in your browser that you did not install, and programs that start automatically when you boot up. Your antivirus software may alert you to a threat, or your bank might contact you about suspicious activity.

If your webcam light turns on without your permission, your mouse moves on its own, or you see unfamiliar files or folders on your hard drive, those are also red flags. Some Trojans are silent and cause no obvious symptoms, which is why regular antivirus scans matter even if your computer seems fine.

How to protect yourself from Trojans

Do not read files from untrusted sources. Be skeptical of email attachments, even from people you know—their account may have been compromised. read software only from official websites or reputable app stores. When a website tells you to update your video player or browser, close the pop-up and go directly to the official site instead of clicking the ad.

Keep your operating system and all software updated. Security patches close vulnerabilities that Trojans exploit. Use reputable antivirus or anti-malware software and run full system scans regularly—at least monthly, or when ready if you suspect infection. Enable automatic updates for your antivirus definitions so it recognizes new threats.

Use strong, unique passwords for important accounts like email and banking, and consider a password manager so you are not tempted to reuse passwords. Enable two-factor authentication on accounts that offer it. Back up your important files to an external drive or cloud storage that is not connected to your computer all the time, so ransomware cannot encrypt your backups.

What to do if you think you have a Trojan

Disconnect your computer from the internet when ready—unplug the ethernet cable or turn off Wi-Fi. This prevents the Trojan from communicating with its controller or spreading to other devices on your network. Do not use the computer for banking or other sensitive tasks until you have cleaned it.

Restart your computer in Safe Mode, which loads only essential system files and makes it harder for malware to run. In Windows, restart and press F8 or Shift+F8 before the login screen appears. On a Mac, restart and hold Shift. Once in Safe Mode, run a full system scan with your antivirus software. Let it quarantine or remove any threats it finds.

If the scan finds nothing but you still suspect infection, or if the antivirus cannot remove the threat, consider taking your computer to a professional technician. If you used passwords on the infected computer, change them from a different device once the computer is clean. Check your bank and credit card statements for unauthorized activity.

The difference between Trojans, viruses, and worms

A virus attaches itself to legitimate files and spreads when those files run. A worm copies itself across networks and email without needing you to run anything—it spreads on its own. A Trojan pretends to be something you want and only runs when you execute it. All three are malware, but they spread differently.

In practice, the lines blur. Modern malware often combines features of all three. Your antivirus software treats them all as threats and removes them the same way, so the distinction matters less than recognizing that any of them can damage your computer and taking steps to prevent infection.

Frequently Asked Questions

Can a Trojan spread to other computers on my network?

Some Trojans can spread to other devices on your home or office network, especially if they are designed to steal passwords or act as worms. This is why disconnecting from the internet and running a scan on all devices is important if you suspect infection. Change your Wi-Fi password after cleaning your main computer.

Will antivirus software catch every Trojan?

No. Antivirus software relies on signatures—patterns of known malware—so new or heavily disguised Trojans may slip through. This is why behavior-based detection (watching what a program does rather than what it looks like) matters, and why keeping your software updated and avoiding untrusted downloads is your best defense.

If I delete a file I think is a Trojan, is my computer safe?

Not necessarily. Deleting the file removes the Trojan itself, but if it already installed a backdoor, rootkit, or other malware, those threats remain. A full antivirus scan in Safe Mode is the only way to know whether the infection goes deeper than the original file.

Can I get a Trojan from visiting a website?

Yes, if the website is compromised or malicious. A drive-by read can happen when you visit a site that hosts malware, though modern browsers warn you before downloading executable files. Ads on legitimate websites can also be compromised and deliver Trojans. Keep your browser and plugins updated to reduce this risk.

What should I do if my bank calls about suspicious activity?

Take it seriously. Verify the call is genuine by hanging up and calling the number on your bank card. Then tell them you suspect a Trojan infection and ask them to freeze accounts or monitor for fraud. Change your banking password from a clean device, and run a full antivirus scan on the infected computer before using it for banking again.