Plague trousers are a pair of trousers obtained from the Sheep Herder quest. It is bought from Doctor Orbon for 100 coins during the quest.
The full set, consisting of the trousers and a plague jacket, grants access to the incinerator area north of East Ardougne. There's not much there except a respawn point of a cattleprod, which cannot be obtained without the set. However, they do not grant access to the Mourners' HQ basement in West Ardougne.
The plague set looks almost identical to Mourner gear before its update. Unlike the Mourner gear, however, this, along with the plague jacket, cannot be used to enter the mourner hideout during and after Mourning's End Part II.
Previously, plague trousers could not be obtained again if dropped after completion of Sheep Herder. This was changed with the release of Quest Point Shop on 24 September2018.