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There was a boot-sector virus on the Atari ST called Cookie Monster. Once in memory, as well as the usual smearing itself across every disk put in the drive, it would appear at n/2^(Number of times popped up) intervals (I think n was about 5 minutes), and say 'I want a cookie'. When the user typed 'cookie', it went away.

Doing the math, after about 4 or 5 popups the machine was effectively unusable.

You know an emulator is good when you see an emulated virus spreading through your emulated disk images...