The Corrupted Blood incident - World of Warcraft
Another event that happened in World of Warcraft was done through the use of the MMOs own design. It happened back in 2005 with the release of the raid zone "Zul'Gurub" who's end boss would cast an ability called "Corrupted Blood" on the players.
At the time, it was also cast on pets which hunters noticed it stuck around with the timer paused when they dismissed their pet. Then the fun happened. They would call their infected pets in the middle of the auction house of the capital cities (where the most players were located) and just watch the fun. When its duration ended, it instantly killed whomever it was attached to and spread to others, including the NPCs! It quickly took over and became an epidemic that couldn't be stopped without Blizzard stepping in to reset the servers and patch the spell from attaching to pets.
It actually made mainstream news at the time because it was considered the first "virtual" outbreak of a disease to ever happen.