Keen Smell: The rat has advantage on Wisdom(Perception) checks that rely on smell.   Pack Tactics: The rat has advantage on an attack roll against a creature if at least one of the rat's allies is within 5ft of the creature and the ally isn't incapacitated.   Filth Fever: This disease has an onset period of 1d3 days, after which the afflicted creature must make a new Constitution save at the start of each new day. On a failed save, the creature takes 1d3 Dexterity damage and 1d3 Constitution damage. This damage cannot be healed until 2 consecutive saves are achieved.