EFFECTS OF THE GREAT PLAGUE

The Great Plague began beyond Mordor in Rhûn. It was believed by historians to have allegedly been caused by Sauron as the plague coincided with a Shadow deepening in Mirkwood, foretelling his return in the unguarded Mordor and the reappearance of evil things.
 
During the cold winter of T.A. 1635, the plague was brought by dark winds to Rhovanion. The Northmen suffered great casualties since they were less skilled in the art of healing than Gondor was. After the plague had passed, more than half of the folk of Rhovanion had been killed along with their horses.

The plague also hit the enemies of Gondor, who otherwise could easily have overwhelmed the weakened kingdom. Though this didn't affect Sauron, as he could wait.

The plague first reached Gondor in T.A. 1636, just a couple of years after King Minardil had been killed at Pelargir by the Corsairs of Umbar. It was devastating in Gondor; Calenardhon and Osgiliath were especially hit hard, with Calenardhon being nearly completely abandoned and Osgiliath suffering the highest casualties outside of Rhovanion. Many fled the city and removed to Ithilien and Anórien, and Minas Anor became the King's seat. The casualties were so high that "Minas Ithil was emptied of its people" and the fortresses guarding Mordor were abandoned as the troops were recalled. The new king, Telemnar, was killed along with his children, many others of the Dúnedain and the White Tree of Gondor. Coming two centuries after the Kin-strife, this further destroyed the people of Gondor.
 
From there the plague spread west and then north, desolating many of the lands in Eriador. Dunland suffered less than most, due to having little contact with other regions. However, the plague gained strength as it went north and Cardolan, Minhiriath in particular, was hit especially hard. The joint garrison of the North and South Kingdoms at Tharbad ceased to exist. The last of the Dúnedain of Cardolan died on the Barrow-downs and evil spirits from Angmar and Rhudaur entered the realm.

While The Shire suffered greatly, the plague lessened as it passed northward until the northern part of Arthedain was scarcely affected, still being capable of defending Fornost Erain from Angmar. The plague marked the beginning of the desolation of Eriador, where the population of Men continued to decline for the rest of the Age. The exact date that the plague ended is not known, but for the next two centuries, Gondor, as well as its allies and enemies, did little but try to regain strength.