Keen Smell. The swarm has advantage on Wisdom (Perception) checks that rely on smell. Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny rat. The swarm can't regain hit points or gain temporary hit points.
Bites (swarm has more than half HP). Melee Weapon Attack: +2 to hit, reach 0 ft., one target in the swarm's space. Hit: (2d6) piercing damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or contract a disease. Bites (swarm has half HP or less). Melee Weapon Attack: +2 to hit, reach 0 ft., one target in the swarm's space. Hit: (1d6) piercing damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or contract a disease.