Most experiments in which biologists — or, more accurately, epidemiologists — study how a disease spreads are done theoretically, involving only a pen and paper. They do their best to simulate the spread of various contagions and study outbreaks of the past, but nobody would dare spread a disease simply to study it.
In 2005, however, they were given the perfect test conditions and subjects: World of Warcraft players.
World of Warcraft is a massively multiplayer online role-playing game made by Blizzard Entertainment set in a fantasy realm called Azeroth. In September of 2005, a new "raid" encounter — an experience that required 20 players — opened up, called Zul'Gurub. This was, basically, an ancient city loosely based on Mayan culture that belonged to a savage tribe of Trolls.
When players finally fought the final boss, the Serpent God Hakkar, they would contract a temporary "debuff" (or weakness) called "Corrupted Blood" that would slowly drain their health before spreading to another player.
Once a player was infected, they'd have to wait out the sickness or die in the process. It wasn't much more than a nuisance to high-level players, as they could simply heal themselves and continue fighting, hoping to pry an epic sword from the Serpent Lord's cold corpse. But the in-game plague didn't just affect players.
In the game, you play one of several different fantasy roles, including paladins, druids, rogues, and (most relevant to this scenario) hunters. Hunters specialized in taming beasts that would then fight in their name. If a hunter's pet contracted Corrupted Blood, the player could "dismiss" their pet, making it effectively disappear. The next time that pet was called to help, however, it would still have the disease — and it would still spread to nearby characters.
Hunters of the world would (sometimes) inadvertently bring their infected pets back to large population hubs after completing the raid. There, they'd call forth their beast without realizing it was still infected. Then, the Corrupted Blood was transmitted to other players outside of the raid. This time, the infected players weren't powerful heroes attempting to kill a god, but rather low-level noobies that would quickly die once affected by the plague, causing it to infect others.
This spread just like a real plague. Players, in search of safety, would evacuate large cities, bringing Corrupted Blood to outlying hamlets, just as with real plagues. Some players would knowingly infect themselves just to harass other players, akin to bio-terrorism.
It was fixed within a week and the game developers apologized for the bug (even though they intentionally recreated it a few years afterwards). But this was the perfect scenario that every epidemiologist dreams of recreating without risking their medical license.
Years after the virtual incident, many researchers published documents using information gathered from the digital plague. They tracked how animals that humans keep as pets might be the most prone to infecting others. They monitored how the disease spread through major population centers and how it traveled along pathways towards the outer reaches of the game. It even simulated surprisingly lifelike actions of bio-terrorists and how they can be dealt with.
All in all, it was a mild annoyance to the players but it gave the Center for Disease Control and many researchers a realistic and ethically-sound testing environment.