Personal: Booted up a friend’s infected disk on my Amiga, which then infected the HD. Mass panic for ten minutes or so as I ran Virus Checker or VirusZ on it.
Work: In 2003-ish we had an infection of… I can’t even remember the name of it, but we had to manually go round and run a program on everybody’s computer to get rid of it.
Since then I’ve seen a few people get their files encrypted by Ransomware, but no major infections.