As Alex explained in the WoW Archivist on the plague, Corrupted Blood was actually a debuff cast on the players, and it spread because hunter and warlock pets could get the debuff, be dismissed, and when summoned again would still have the debuff on them. This allowed those players to release the Corrupted Blood debuff into heavily populated areas where it wiped out hundreds in short order, especially lower level characters who didn't have anywhere near the health pools to endure the disease. The debuff didn't have a very long duration otherwise, and so it usually ran its course or killed people before they could leave Zul'Gurub with it. But creative players found a way to use their pets to become architects of destruction on a wide scale.
I personally remember a friend back in these days, when I raided on Azjol-Nerub, who managed to kill Orgrimmar with his pet and Eyes of the Beast. (Hey, Tyr. Still miss you, dude.) He ran that thing into a crowd of Horde players outside the old AH and from there, it was pandaemonium. It actually ended up being so bad that the entire server ended up shut down from multiple pets with the debuff turning Ironforge (back then the only place with an AH) into something from a Romero film. So let's all take a moment and remember the Corrupted Blood plague of September 13th, 2005 on this, its seventh anniversary.