Late last night, reports WoW Insider, Death Knights in World of Warcraft contracted a bug which allowed them to cast plagues upon friendly targets. The afflicted death knights, as per their namesake, went apeshit with this newfound power, laying waste to Azeroth's population in a way not seen since the infamous "Corrupted Blood" plague—which celebrated its seventh anniversary on Thursday.
The Death Knight bug was patched out very early this morning, WoW Insider said, but what makes it different is the fact this plague couldn't spread by itself. It required action by the knights themselves. It still was the worst epidemic seen since Corrupted Blood, a 2005 incident later studied by epidemiologists.
Here's an image from the outbreak, via WoW Insider; there are four more at the link.
Death knight plague epidemic strikes Azeroth [WoW Insider]