The Corrupted Blood Incident
Thanks to the cautionary tales podcast I learned about the corrupted blood incident on World of Warcraft. It turns out that in 2005 the massively online videogame was struck with a transmissive disease that led to a pandemic in Azeroth.
The disease, corrupted blood, was a spell from a raid boss in Zul'Gurub. It would drain the life out of a player and only high-level players would survive. Unfortunately the disease could spread to other players, as well as NPCs and the players pets. After a few infected players started leaving the dungeon to go back to a capital city it started killing off all low-level players who'd be walking around.
The disease spread so quickly that the only way to kill this pandemic was to reset the servers.
The aftermath was pretty interesting too. The US Centre for Disease Control asked Blizzard for data, which they declined to give. But some academics also wrote comments about the incident, as well as some mayor news outlets.