It was infectious to players and it broke out. Dr. Nina Fefferman is an epidemiologist and she claimed it provided quite a 'fascinating insight' into epidemics.
It was a mistake on Blizzard's part that allowed characters to escape Zul'Gurub while infected. Those infected were meant to essentially die by exploding in blood, but some got out and the 'contagion' spread like wildfire across the world of Azeroth.
"World of Warcraft had at the time a really unique demographic composition. It wasn't the stereotypical gamer," said Dr. Nina Fefferman an assistance professor at the Department of Ecology, Evolution, and Natural Resources at Rutgers University.
Some players began to purposefully infect others, particularly rival guilds, and low level characters were suffering the most. "People were running around exploding into blood, dropping dead and turning into skeletons," said Dr. Fefferman.
Blizzard attempted an imposed quarantine to halt the spread of the outbreak but it failed. Eventually they had no choice but to reset the server to end it. It mimicked how the real world would likely react to such a horrific outbreak of a plague she said.
"We saw some courage... we saw some fear, suspicion of a quarantine," said the doctor. Amid any outbreak the one thing that's most dangerous of all is human behaviour she argues. "People are actually very difficult to predict," she said. "Behavior... that part is really hard." Mathematical models are no match for human motivations.