In 2005, Blizzard added a new boss with a hit-point draining spell that effected anyone standing directly next to him. Since stepping to the boss meant you were probably about to die anyway, they saw no harm in making the spell contagious. The only explanation is that Blizzard had never been on the Internet, and therefore had no clue that basic humanity transforms from "Don't share this poop-filled video with anyone because it's horrible" to "THIS IS ABSOLUTELY HORRIFIC I MUST SHARE IT WITH AS MANY PEOPLE AS POSSIBLE!" Players soon worked out how to teleport the plague out of the dungeon and into the real (fake) world. An MMORPG pandemic was born.
One that resulted in more unconvincing skeletons than Jason and the Argonauts.
The hit-points it took away were enough to instantly-kill low level players, so high-level players immediately started teleporting around the map as much as possible. Because if there's one thing World of Warcraft players hate more than people who don't play, it's people who do play but not as much as them. The Breakfast Hen has more respect for its young than online gaming culture, and it's an imaginary bird which lays eggs directly onto a frying pan.
The plague killed new players, old players; it even infected non-player characters who couldn't get sick but acted as carriers -- so talking to an innkeeper about killing 10 wolves could infect and kill you. But after your 20th "Lo, noble warrior, kill exactly 10 pests for me like an OCD Orkin man," that was probably a relief.
"Nah, I think I'll just lay down and accept my fate, thanks.
It was also revealing: In a game where people can be heroic knights or masterful mages, many leaped at the chance to become Terrorist Tyhpoid Mary. A small Taliban like force of plague-carriers actively fought Blizzard while hiding in the mountains, breaking quarantines and even incubating the plague through server-purges by infecting their own virtual pets then re-infecting themselves. They forced Blizzard into hard server resets, nuking and reinstalling their entire world. It was douchebaggery on measurable scientific and national security scales: Real-life scientists and bioterrorism experts now study it as a case example. Presumably before drinking themselves into unconsciousness when they realize they're protecting a species that commits bio-terror in order to destroy worlds they hang out in for fun.
Long live Bonersling420.