The Plaguelands, formerly known as the Eastweald, are the regions of northern Lordaeron that were corrupted by the Undead Scourge during the Third War. It is split between the Western Plaguelands, formerly the fertile lands of the Darrowmere Fields, and the Eastern Plaguelands, which was once known as the majestic Darrowmere Forest.
The Cult of the Damned, under their leader Kel'thuzad, unleased the plague of undeath on an unsuspecting Kingdom of Lordaeron, tainting the grain from Andorhal, the main distributor of grain in the northern regions, and shipped it to many outlying towns and villages. The plague killed thousands of people, raising them into undeath under the command of the Lich King, and the plague spreaded, tainting the land itself. The plague was temporarily halted when Prince Arthas Menethil and Uther the Lightbringer were able to destroy the plagued granaries, although by the time King Terenas was killed, barely a single living human was left in Lordaeron, and the lands had been tainted from what is now known as the Ghostlands all the way to the Silverpine Forest, although the Ghostlands were burned down for the most part.
The region was divided after the Scourge's defection from the Burning Legion and the splintering of the Forsaken under Sylvanas Windrunner. While the Forsaken managed to relatively secure the Tirisfal Glades, the Scourge continued to further corrupt the eastern-most lands. The Eastern Plaguelands' taint is such that mushrooms started to grow, both in the wildlife and on it, such as on many of the trees.
Other than forces of the Grand Alliance and of the Forsaken, the Scarlet Crusade also fought the Scourge, although they were also known to attack anyone who may have seemed to oppose them. However, they were successful for a time, and managed to retake the towns of Tyr's Hand, Hearthglen, the western section of Stratholme, the Tirisfal Monastery and several other small towns. Also present in the Plaguelands was the Argent Dawn, which was primarily composed of ex-Scarlet Crusaders. From the Light's Hope Chapel they struck against the Scourge, although they didn't attack strangers like the Crusade did.
For a long time, the Dread Citadel Naxxramas floated above the Plaguewood, until the Lich Kel'thuzad's fall at the hands of the champions of the Argent Dawn. Because of a traitor within the Dawn's own ranks, he was resurrected, and the necropolis was called to Northrend.
Today, much of the Western Plaguelands has been healed by the combined efforts of the Argent Crusade and the Cenarion Circle, and those efforts carry on to the Eastern Plaguelands. Although they are far from being done, perhaps one day the lands will be restored to their former glory.