system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do antivirus programs see what is a virus and not?
gpt
The simplest kind just compares against a database of known malware. If it fits, it's malware. (Probably.)

More advanced detection uses heuristics that try to determine whether an action being taken by a program or whether certain qualities are nefarious in nature. This allows for currently unknown threats to be shut down, but has a much higher false-positive rate.