system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I understand how you can get viruses from surfing porn, torrent sites etc, but can someone explain to me what happens when Microsoft Security Essentials (or similar) catch threats when I'm surfing "safe" sites?
gpt
It means the threat is already on your computer, and is only activating now, due to random timing, or to influence your actions on this site (e.g logging keystrokes).

Alternatively, the "safe" site may be infected. Browsers like Chrome will warn you when you attempt to access an infected site however, so this is less likely.