Corrupted Blood may be the most interesting fake disease on this list. In many ways, this is one of the only computer related viruses that was never intended to exist and its natural response played out very closely to that of a real viral outbreak. In 2005 the MMORPG World of Warcraft released a new dungeon where users would fight the God of Blood. One of the creature's attacks was a spell that when cast would cause the affected character to slowly drain hp and would weaken all other stats. On top of that it would spread to anyone that character came close to. It was originally intended to only work while in the confines of the new dungeon...but the programmers neglected to consider what would happen if one would teleport while infected. It turns out that an infected player could take the disease with them and when put in a heavily populated area caused a spread that was almost unable to be contained. This spread was done both by accident and intentionally by more chaotic minded individuals.
This became a virtual plague that quickly killed off low level characters and drastically hindered higher level ones. The virus-spell was even able to spread to non-player characters causing them to host the disease while displaying no symptoms...thus infecting players who would talk to them to obtain quests. Programmers initially tried to contain the virus by creating quarantine zones, but many did not take the precautions seriously and others purposfully got around them by giving the disease to summonable pets.
Many others still decided to leave populated cities while they could, while those that stayed or spawned would be added to the sea of corpses with the streets literally turning white with bones of the dead. In the end none of the attempts to allow the virus to play out its natural course were successful and the Corrupted Blood glitch was only defeated by a series of patches and a reset of the virtual world.