Patient Zero

Patient Zero is a Legion raid achievement earned from killing Helya while all raid members but one are afflicted with Fetid Rot, in Trial of Valor on Normal difficulty or higher.

Fetid Rot comes from the Grimelord that spawns in phases 1 and 2. It is best for two players to bounce the debuff between each other, and then share it with the rest of the raid bar one when Helya is on low health.