When most people think of computer viruses, they picture a skull popping up on screen or files being deleted one by one. That was the 1990s. Modern malware is designed to be invisible. It doesn't want you to know it's there, because the longer it runs undetected, the more money it makes for the attacker.

If your computer is obviously acting strange, the problem is usually something else (a rogue app, adware, or just too many startup programs). Real malware hides.

Types of modern malware

Info stealers

The most common and dangerous type right now. Info stealers silently grab your saved passwords, browser cookies, credit card numbers, and cryptocurrency wallets, then send everything to the attacker. You won't notice anything unusual. By the time you find out, your accounts are already compromised. See what to do if your account is compromised.

Keyloggers

Software that records every keystroke you make – passwords, messages, credit card numbers, everything. Some advanced keyloggers also capture screenshots at regular intervals. They run silently in the background.

Cryptominers

Malware that uses your CPU and GPU to mine cryptocurrency for someone else. Your computer runs hot, fans spin constantly, and performance tanks. If your computer is suddenly overheating or running slow for no reason, an unwanted cryptominer is one possibility.

Remote access trojans (RATs)

These give an attacker full remote control of your computer. They can see your screen, use your webcam, browse your files, and install additional malware. RATs are often disguised as legitimate software or delivered through phishing emails.

Trojans

Any malware that pretends to be something useful. A free game, a "PDF viewer," a cracked version of paid software – you install it thinking it's legitimate, and it installs malware alongside (or instead of) what you expected. This is the most common delivery method for all the malware types above.

Botnets

Infected computers that are silently recruited into a network controlled by an attacker. Your computer might be used to send spam, launch attacks against websites, or spread malware to others – all without your knowledge. The main symptom is increased network activity and slower performance.

How you get infected

• Downloads from sketchy sites – Free software from untrustworthy sources, especially cracked or pirated software, is the number one way malware spreads
• Pirated software and games – If you're downloading paid software for free, you're almost certainly getting malware with it. The crack itself is often the malware
• Email attachments – Files from unknown senders, especially .exe, .zip, .scr, and .js files. See avoiding phishing
• Malicious ads (malvertising) – Ads on websites (even legitimate ones) that redirect to malware downloads. An ad blocker helps here
• Fake software updates – Pop-ups claiming your browser, Flash Player, or Java needs updating. Your browser updates itself. Flash Player doesn't exist anymore. These are always fake
• Infected USB drives – Less common now, but plugging in an unknown USB drive is still risky

Do you need antivirus?

This is the most common question, and the answer depends on your platform.

Windows: Windows Defender (built in, called "Windows Security") is genuinely good now. Independent testing labs consistently give it top marks for malware detection. For most home users who practice basic caution – not downloading pirated software, not clicking suspicious email links – Defender is enough. You don't need to pay for a third-party antivirus.

Mac: macOS has built-in protection through XProtect (scans for known malware) and Gatekeeper (blocks unsigned apps). These update automatically in the background. Macs are not immune to malware, but the risk is lower if you stick to the App Store and trusted developers. Most Mac users don't need additional antivirus. If you want extra coverage, Malwarebytes (free version) is a solid option for occasional scans.

Linux: Malware targeting Linux desktops is rare. Most Linux users don't run antivirus. Keep your system updated and don't run random scripts from the internet.

All platforms: Common sense is the best antivirus. Don't download software from shady sources, don't open unexpected email attachments, keep your OS updated, and use strong, unique passwords with a password manager.

Signs your computer might be infected

Malware tries to stay hidden, but here are signs something might be wrong:

• Your computer is significantly slower than usual without an obvious cause
• Fans run constantly even when you're not doing anything demanding
• Programs crash frequently or behave oddly
• Your browser redirects to unexpected websites (this is more likely adware)
• You see processes in Task Manager (Windows) or Activity Monitor (Mac) that you don't recognize and that use significant CPU
• Friends receive strange messages or emails from your accounts
• Your antivirus has been disabled and you didn't do it
• New programs appear that you didn't install

Note: many of these symptoms can also be caused by non-malware issues. A slow computer is more often caused by too many startup apps or a full hard drive than by malware.

What to do if you think you're infected

1. Run a full malware scan – Use Windows Security (Windows) or download Malwarebytes (any platform) and run a full system scan
2. Change your passwords from a clean device – If malware is on your computer, don't change passwords from that computer. Use your phone or another device. See compromised account response
3. Enable two-factor authentication – On all important accounts, especially email and banking. See two-factor authentication
4. Update your operating system – Install all available updates. This patches the vulnerabilities malware exploits
5. Check your accounts – Look for unauthorized transactions, password changes, or new devices logged into your accounts
6. If the scan finds something – Let the antivirus quarantine or remove it. Restart your computer and scan again to make sure it's gone
7. If it keeps coming back – You may need a more thorough cleanup. Boot into Safe Mode and scan again, or consider a clean OS reinstall as a last resort