Several instructors were introduced to the school, but one of the main necromancy teachers was a skeletal mage named Vectus -- and Vectus had a plan. While the plague cauldrons had been moderately successful in spreading the plague across Lordaeron, the Scourge needed to engineer something far more powerful to spread the plague further. Players doing quests out in the Burning Steppes can discover the first indicator of this plan -- a goblin named Tinkee Steamboil is doing some special research up at Flame Crest and asks players to help her.
What kind of research? Why, on dragons, of course – the black dragons that riddle the peaks of Blackrock Spire. Of course it'd be a good idea to help her. The black dragons are bad, after all, evil and intent on wiping out Azeroth, and besides, Tinkee says that her patron will very much appreciate all the hard work you've done and reward you suitably for your efforts. Go on, help her out -- after all, getting rid of a few dragon eggs ought to be helpful, and the frozen eggs ought to be remarkably helpful in her patron's research.
Of course her patron is none other than Vectus. Dragons are awfully powerful creatures, and if say, they were injected with a plague that didn't kill them, simply made them airborne carriers of the thing, that would be much easier than lugging cauldrons of the stuff around -- especially since that pesky Argent Dawn figured out a way to neutralize those cauldrons and were diligently working away at it.
Speaking of the Argent Dawn, if you do decide to help out Tinkee (and why not, xp is xp after all), she'll ask you to deliver a crate of frozen dragon eggs not to Vectus, but to another man who sent her a message on behalf of Vectus -- his name is Leonid Barthalomew. Not only is he not working for Vectus, he's one of the higher ranked members of the aforementioned Argent Dawn. Players arriving at Light's Hope are told to report to Betina Bigglezink, who promptly asks them to go to Scholomance and destroy a mess of hatchlings. Plagued hatchlings, of course.
And so the plagued dragonflight was created -- not by dragons and breeding, but by taking black dragon eggs and injecting them with the plague. The resultant dragons are still living dragonkin, but not really intelligent from what's been seen. So far, none of the hatchlings have made it to drake status, and there are no plagued drakonids to be seen either.
The Plagued Dragonflight holds the distinction of being the only dragonflight that wasn't specifically bred or created by the dragons themselves -- they were engineered by the Cult of the Damned to spread the plague. While the mission in Scholomance was largely unsuccessful due to player interference, the plagued dragons live on... sort of.
Plagued dragons have returned in Wrath of the Lich King in the form of plagued proto-drakes. Not to be confused with the plagued drakes in Howling Fjord -- a result of the Forsaken meddling with their own plague, these proto drakes are much farther north. Oddly, the Cult of the Damned appear to have nothing to do with this latest incarnation; rather it's the vrykul that serve the Lich King that are using the things as mounts. They're first seen over in the Stormpeaks -- the vrykul of Brunnhildar Village are pitted against the vrykul of Valkyrion, who have decided it would be a really amazing idea to swear to serve the Lich King.
The plagued proto drakes appear again in the village of Ymirheim, located in Icecrown and populated by the Ymirjar -- the elite warriors of the Lich King. Chained to posts around the village, it's apparent that these drakes too are simply for riding, rather than spreading the plague. While this is speculation, I'm guessing that despite Vectus' failed efforts in Scholomance, the technology for creating plagued drakes lived on and was passed to the vrykul in return for their obedience to the Lich King.
One thing to note, however -- while the Lich King and the Scourge are apparently responsible for the development of the flight, the plagued dragons have nothing to do with the frost wyrms that the Lich King pits against players in various dungeons and zones. While the frost wyrms are skeletal dragons that were quite literally raised from the dead, the plagued drakes are living dragons that have simply been overrun with the plague.
The Plagued Dragonflight, while interesting, was ultimately a failed experiment, and the drakes themselves lack the intelligence to prove any kind of a definitive threat to the rest of Azeroth. Indeed, the rest of the flights have paid them little to no attention, though whether it's due to the lack of prowess the plagued dragons hold, or the plethora of problems plaguing (pun fully intended) the other flights is unknown. I have to admit from a lore perspective I'd find it interesting to see a full-grown Plagued Dragon, one with the intelligence to realize what was done to it. I can only imagine it'd be severely annoyed, and the repercussions would be entertaining to say the very least. Given the resurgence of the Black Dragonflight however, this is little more than a wistful dream -- the dragon flights have much bigger things to worry about.