The incident began with Blizzards release of patch 1.7.0 which included the new Zul’Gurub raid featuring end-boss Hakkar the Soulflayer. Due to a bug, a debuff casted by Hakkar was able to leave the raid area and travel to the more densely populated areas of World of Warcraft, killing many players that weren’t strong enough to resist its effect. The event takes its name after the debuff that Hakkar cast, Corrupted Blood.
This event was the first online pandemic and the aftermath of this event would go on to be studied by counter-terrorism researchers and epidemiologists as a way to better understand human population reactions to real world pandemic. 
On the 13th of September 2005 World of Warcraft creator Blizzard introduced a brand new raid called Zul’Gurub alongside patch 1.7.0. The raid’s end-boss Hakkar the Soulflayer was given a number of new spells and debuffs, one of which was Corrupted Blood. This debuff was an instant cast that dealt between 875 and 1125 points of damage but also continued to damage the player at 200 health points every 2 seconds for 10 seconds. Even worse, the Corrupted Blood debuff also had a contagious element that allowed it to jump to other nearby players. High hit-point class players at the time had between 4000 to 5000 total hit points.
Hakkar was a powerful raid boss and players had to devise a number of strategies to survive the critical moment when Corrupted Blood was cast. The most effective strategy was for the player that was infected to quarantine himself away from his clan mates so as to not spread the debuff to his clan mates. 
Debuff’s cast by raid bosses were always limited to the raid areas that they were cast in. If a player left a raid area while the debuff was active, the debuff would immediately dispel. Unfortunately, due to an oversight by the development team, pets who perished while infected with the debuff, retained the debuff after they were summoned again.
At the time, it was common for raid groups to return to major cities in order to resupply between wipes. In the city they would restock their items, repair their armor, and summon their pets. It was at this point that the glitch was discovered, with a summoned pet appearing with the contagious debuff still infecting it. Transmittable between players within close proximity to each other, it was immediately fatal to lower level characters whilst causing confusion among higher level characters.
Initially, there was mass confusion amongst the player base as people couldn’t understand how they were being killed in protected areas. Players would randomly see blood gushing from their character followed quickly by death. At first players were running back to the city centers, resurrecting, only to find themselves fall victim of the plague again. The pandemic was cyclical as entire cities would be laid waste to before partially recovering and then the process would begin again killing entire populations.
Towns and cities became eerily desolate places except for piles of diseased corpses lying in the streets. Regular gameplay was widely disrupted as players ran for unpopulated areas in hopes of avoiding infected players.
As news spread through the userbase about what was happening, so did people’s understanding on how to remove the debuff. Healers offered to stand guard in cities and heal players as soon as they were infected in hopes of quelling outbreaks early. These efforts generated significant interest from the community but were largely ineffective as the viral nature of the debuff was to fast to stop.
Higher level players who couldn’t heal helped by spreading word to lower level players what was happening and leading them away from the infected areas. Others simply isolated themselves in the wilderness where it was unlikely to run into other characters and waited for Blizzard to issue a fix.
A last group of players, dubbed “virtual terrorists” by the gaming press & commentators at large, tried to drive infection of the disease by purposely having their pets infected and summoning them in densely populated areas.
Ultimately Blizzard were forced to implement a series of hard server resets alongside the application of quick fixes in their desperate bid the thwart the illnesses spread. 
At the time of the outbreak of Corrupted Blood, WoW had in excess of 2 million active players based all around the world.During the initial days of the outbreak and before creators Blizzard publicly commented on the incident, there was much debate about whether it was a deliberate programmed decision or whether it was a glitch or programming error. In online forums and fans pages it certainly caused a fair amount of interest and excitement and some players felt that the pandemic represented the first real ‘worldwide’ event within the game’s narrative. Other members of the community were angered that their game-play experience was being damaged by such a glaring error.