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...