The Plague of Undeath is a magical affliction first conjured by Ner'zhul, the Lich King, from his icy prison within the Frozen Throne in Northrend. Using his will alone, he spread the mystic disease to the nearby denizens in the local regions, including Ice Trolls, Nerubians, and other creatures, immediately killing them and causing them to rise again as undead. After the wizard Kel'thuzad answered Ner'zhul's psychic call, he was tasked with containing the plague and transporting it across the sea to Lordaeron, where it was used to infect food supplies and distributed to the populace under the supervision of the Dreadlord Mal'ganis, acting lieutenant of the Burning Legion, Ner'zhul's captors. Kel'thuzad first had to gather a cult of followers to aid him in the spreading of the plague, and he established the Cult of the Damned.
With the aid of his cultists, Kel'thuzad and Mal'ganis used the agricultural hub of Andorhal to ship plague-infected grain to the surrounding towns within Lordaeron, including Brill and Stratholme. Those contracting the plague typically die within three days, rising shortly thereafter as feral zombies easily controlled by the Lich King's immense psychic power. After successfully destroying the infected granaries, Prince Arthas, fresh on the heels of Mal'ganis, came to Stratholme to find it already infected by the tainted grain. Knowing that the citizens would soon turn into the cursed undead, he proceeded to slaughter them all in order to prevent them from bulking Mal'ganis' ranks.
Before becoming a Death Knight in the servitude of Ner'zhul, Arthas succeeded in destroying all of the sources of the plague, leaving no way for them to further spread the disease. A few years later, after the Scourge had built a considerable presence in the Plaguelands, they set up a series of massive cauldrons with which they continue to spread the plague directly into the air, corrupting the land itself as well as the local wildlife.
The Argent Dawn, a neutral regime dedicated to the eradication of the Scourge menace, have discovered ways of sabotaging these cauldrons and reversing their effects, thereby purifying the chemicals within and making them toxic to the undead.
Currently, Apothecaries of the Forsaken are working on producing their own New Plague, which would have a similar effect as the original Plague, but with far deadlier results, first turning its victims into ghouls before killing them mere seconds later. Experiments on this New Plague have been conducted by Apothecaries in Brill, the Sepulcher, and Tarren Mill.
The Undead Plague
From Undead Plague
The devastating undead plague began in Northrend after the Second War. There, from the depths of the Frozen Throne, the Lich King Ner'zhul afflicted a remote human village through his will alone: a morbid test meant to gauge the plague's effectiveness. The infected villagers died, and when their zombified corpses rose soon after, they had become lumbering, mindless servants of Ner'zhul.
The experiment was successful, but the Lich King was interested in nothing less than perfection. He contaminated every human inhabitant of Northrend, binding them to his icy will even as he continued to fine-tune his infernal disease.
Infested Plaguelands Through the course of his experimentations, Ner'zhul insured that the affliction would specifically target humans for “undeath”. Though non-human races and creatures (and even the land itself) were susceptible to the plague, it was humanity in particular that Ner'zhul meant to scour from the world. As a result, infected flora and fauna reacted differently--diseased and decaying, but not truly undead, and not under the thrall of the Lich King.
Hence, while undead representatives certainly do exist among the ranks of the non-human races, these particular agents are examples of undead created through necromancy rather than the plague.
Once Ner'zhul's adjustments were complete, his mind reached out to Dalaran, to the disgruntled human, Kel'Thuzad. The archmage answered the call, trekking through the arctic wastes of Northrend to eventually climb the steps of the Frozen Throne*. There he pledged to act as the Lich King's lieutenant in exchange for immortality and untold power. He was then given cauldrons of concentrated plague to spread throughout the lands of Lordaeron via his acolytes in the Cult of the Damned, and soon dead villagers throughout the realm began to rise and walk again, marching against the living in obedient servitude to their new master.
And thus, the Scourge was born into an unsuspecting world.