Infection. Whenever the swarm hits a creature with a melee attack, that creature must succeed on a DC 12 Constitution saving throw. On a failure, the damage from the attack lowers the creature's maximum hit points. The hit point maximum can not be restored unless either a creature succeeds on a DC 12 Wisdom (Medicine) check, or something similar to the spell Lesser Restoration or a paladin's Lay on Hands feature is used to cure the infection. If a creature's maximum hit points drops to zero, the creature dies & rises as a zombie in 2 (1d4) days.

If a creature succeeds on the initial saving throw, it is immune from this swarm's infection for the next 24 hours. This feature does not affect constructs or undead.

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 cat. The swarm can't regain hit points or gain temporary hit points.

Undead Fortitude. If damage reduces the swarm to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the swarm drops to 1 hit point instead.