Corrupted Blood

The 'Corrupted Blood was a virtual "plague" that occured in World of Warcraft. It is notable as it resembled a real life epidemic.

Origins
In September 13, 2005 (hint hint, 13? just kidding) Blizzard released a new instance (a place where players band together to fight high level monsters) called Zul'Gruub. In that instance, the final boss, Hakkar the Soulflayer, could cast a debuff (spell that has a negative effect over time) called corrupted blood. It would deal heavy damage to whoever it was on and could also spread to people, npcs, or even pets nearby. Although Blizzard never intended the debuff to leave the instance, naturally, the players wanted to go back to the major cities...

The Disease Takes its Toll
The plague, sometimes compared to the Black Death, spread rapidly, reaching pandemic sizes within days. Low levels were killed instantly, but higher levels could stay alive long enough to pass the plague on to new victims. Wiser players sometimes avoided all major cities completely, as those were the centers of the corrupted blood. In some cases, infected players were ostracized by others in a vain attempt to keep the healthy safe. In some servers, nearly half of the population had the corrupted blood. Because of World of Warcraft's huge size, and the equally large scale of the pandemic sweeping it, the Corrupted Blood Plague is likely to have been the only video game glitch in history to have recieved wide attention from mainstream media.

Cure at Last
When Blizzard realised that their former debuff turned the servers into lands of disease, they tried several times to stem the corrupted blood. They had only intended it to complicate player tactics, not to wipe out entire cities. The staff tried several times to cure pandemic, including quarantining certain players. Eventually, they managed to cure the plague for good by simply removing the corrupted blood debuff's ability to spread.

Interest from Medical Groups
While we can safely say that the players were not too thrilled about the Corrupted Blood Plague, it has been hailed by medical experts as a way to predict the patterns of the potential plagues like avian flu and influenza. In fact, some have even called for more of these pandemics to occur on online games, wishing to study the effect. Naturally, the online game players responded with complete disgust. It would be possible, though, to add a disease that does nothing but spread, simply to see how it spreads.