Sewer Rat

You come from a long line of ysoki from a community based in the sewers beneath a large settlement. You are immune to the disease filth fever. Each of your successful saving throws against a disease or poison reduces its stage by 2, or by 1 for a virulent disease or poison. Each critical success against an ongoing disease or poison reduces its stage by 3, or by 2 for a virulent disease or poison.