Equip: Increases your critical strike rating by 14.
Sell Price: 3 15 42
The Leggings of the Plague Hunter is a quest reward from the Argent Dawn faction.
Source
The quests are given by Quartermaster Miranda Breechlock. She has a Quest for each tier of faction (Friendly, Honored, Revered and Exalted). It requires a turn-in of a large number of Insignia of the Dawn and Crusade. (See Armaments of the Dawn for summary).
The Quests are: