Assimilated brown bear

Keen Smell. The bear has advantage on Wisdom (Perception) checks that rely on smell.

Fire aversion. Any fire damage dealt to the assimilated bear removes its ability to transmit the plague.

Concealment. The bear can use 2 actions to reform itself to make it appear as if it isn't infected.

Multiattack. The bear makes two attacks: one with its bite and one with its claws.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (1d12 + 4) piercing damage. The target has to make a DC 11 constitution saving throw or become infected with the plague.

Claws. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.