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Guide
What does antivirus software actually do?
The confusion
Antivirus products describe themselves as protecting against malware, threats, viruses, hackers, ransomware, spyware, and 'online dangers' — often all in one sentence. The actual mechanism behind any of that is never explained. You're supposed to trust that the product handles it.
Then you read that antivirus 'doesn't catch zero-day threats.' Or that it 'can't stop phishing.' Or that 'most breaches bypass antivirus entirely.' These statements contradict the protection claims without explaining what the actual gap is.
Understanding what the software is actually doing — not the marketing version — changes how you evaluate products and what you expect from them.
What most people assume
Most people assume antivirus works by recognizing viruses the way a face recognition system recognizes people — by matching what it sees to a database of known threats. That's partly true for signature-based detection, and it's where the 'can't catch zero-days' limitation comes from. But modern antivirus products also run behavioral monitoring: watching what processes do in real time, flagging actions that look like malware behavior regardless of whether the file has been seen before.
Most people assume antivirus scans files before they cause damage. Some protection happens at that stage — files are checked on download or execution. But a significant amount of modern detection happens while code is already running. Behavioral detection and sandboxing catch threats in the act of doing something suspicious, which means some level of execution happens before anything is blocked. 'Blocked before it ran' and 'blocked while it was running' are different things.
Most people assume antivirus protection is a single layer — either it stops the threat or it doesn't. In practice, modern products stack multiple detection methods: signature matching, heuristics (pattern-based guessing on unknown files), behavioral monitoring, cloud-based lookups, and sandbox execution for suspicious files. Any one method has gaps. The combination is designed to compensate for individual weaknesses.
What's actually true
Antivirus software runs continuously in the background doing several distinct things: scanning files as they arrive (on download, on copy, on execution), monitoring active processes for behaviors associated with malware, checking files and URLs against cloud-based threat databases in real time, and quarantining or terminating anything that matches a threat pattern. The scanning you see when you run a manual check is the most visible part of a system that's working across multiple layers most of the time.
The gaps are structural, not product-specific. Signature detection can only catch threats already in the database. Behavioral detection produces false positives and can be evaded by malware that mimics legitimate behavior. Cloud lookups require a connection and a database that's already been updated for the threat. No combination of these methods catches everything — the question is how wide the gap is and how quickly new threats enter the database after discovery.
Where you might be
If you're trying to understand whether your current antivirus is actually doing anything — or whether it's redundant with something else already running — the answer starts with which detection layers are active and whether they overlap with Windows Defender.
See how Defender and third-party AV compare in practice →If you're evaluating products and want to understand what the test scores actually measure — why one product scores 99.7% and another scores 97% and what that difference means in practice — that distinction lives in which detection layer is being tested.
See how to choose based on your actual threat exposure →
If your machine is already behaving strangely and you're wondering whether your antivirus should have caught whatever is there — the gap between what antivirus can and can't catch explains why it might have missed something.
See the cleanup path for an already-infected machine →If you're specifically trying to understand ransomware protection — whether antivirus catches it and what rollback actually means — that's a specific capability separate from general malware detection.
See how ransomware works and what protection covers →What no tool solves
Antivirus doesn't scan encrypted traffic. If malware is downloaded over HTTPS, the content isn't visible to the antivirus until it arrives on disk or tries to execute. The network layer is mostly opaque to antivirus — which is why behavioral detection of what happens after download matters.
Behavioral detection generates false positives. Legitimate software that behaves like malware — installers that modify system files, development tools that inject into processes, backup software that reads the entire disk — can trigger blocks or quarantine actions. This is a deliberate tradeoff, not a malfunction.
Antivirus operates on what's happening on your device. It has no visibility into what you type into a phishing page, what you agree to in an installation prompt, or what you share over a messaging app. The human decisions that happen in a browser or in conversation are outside the protection surface entirely.
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