Pestilence Swarm

Dormant in Darkness. The pestilence swarm appears dead in darkness and remains in a passive state until touched by light. When at least one insect in the swarm sees light, the entire swarm becomes active and attacks anything that moves. Once active, a swarm in complete darkness that isn't taking damage returns to its dormant state after 1 minute. The swarm poses no threat to creatures passing near it in complete darkness.

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 beast. The swarm can't regain hp or gain temporary hp.

Diseased Bites. Melee Weapon Attack: +5 to hit, reach 0 ft., one creature in the swarm's space. Hit: 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half its hp or fewer. The creature must succeed on a DC 11 Constitution saving throw or contract sewer plague. It takes 1d4 days for sewer plague's symptoms to manifest in an infected creature. Symptoms include fatigue and cramps. The infected creature suffers one level of exhaustion, and it regains only half the normal number of hp from spending Hit Dice and no hp from finishing a long rest. At the end of each long rest, an infected creature must make a DC 11 Constitution saving throw. On a failed save, the creature gains one level of exhaustion. On a successful save, the creature's exhaustion level decreases by one level. If a successful saving throw reduces the infected creature's level of exhaustion below 1, the creature recovers from the disease.