Assassinate. During its first turn, the doctor has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the doctor scores against a surprised creature is a critical hit.

Plague. When a creature is exposed to plague, it must make a DC 15 Constitution saving throw against disease or become poisoned until the disease is cured. Every 24 hours that elapse, the target must repeat the saving throw, reducing its hit point maximum by 5 (1d10) on a failure. The disease is cured on a success. The target dies if the disease reduces its hit point maximum to 0. This reduction to the target's hit point maximum lasts until the disease is cured. 

Plague Carrier. When the doctor is reduced to 0 hit points, or its mask is removed, any creature within 10 feet of it is exposed to plague.

Poison Absorption. Whenever the doctor is subjected to poison damage, it takes no damage and regains a number of hit points equal to the poison damage dealt.

Walking Sickness. If the doctor is targeted by an effect that cures disease or removes a curse, the doctor is stunned until the end of its next turn.

Multiattack. The doctor makes two syringe attacks.

Syringe. Melee or Ranged Weapon Attack: +6 to hit, range 20/60 ft., one target. Hit: 5 (1d4 + 3) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much damage on a successful one. When the plague doctor scores a critical hit on the attack roll, the target is also injected with a full dose of plague.

Swarm. When the doctor takes at least 12 damage from a single weapon attack, it can choose to take twice that amount instead and summon a swarm of rats with a number of hit points equal to the damage taken. The swarm appears in the plague doctor's space and acts on its own initiative.