Undead Fortitude. If damage reduces the zombie 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 zombie drops to 1 hit point instead.

Scratch and Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage. If the target is a creature, it must succeed on a DC 10 (+1 for every additional failed save of becoming Infected by Plague Zombies) Constitution saving throw against disease or become infected until the disease is cured. After every Short or Long Rest, the infected creature must repeat the saving throw, reducing its hit point maximum by 1 for each time the creature was infected, on a failure. This reduction lasts until the disease is cured. The creature turns into a plague zombie, if the creature's hit points is reduced to zero 0 HP. a Paladin's Lay on Hands (5 hp spent) or Lesser Restoration Spells will remove the zombie plague disease from the infected creature.

Loud Noises. The sounds of Combat or Yelling may cause 1D4+1 Plague Zombies to appear near the noisy area on the next round. Roll 1d6 on 5 or 6 will spawn more Plague Zombies. Spells like Thunderwave, Thunderclap, Thunder Step, etc that has a description such as, "can be heard 100 feet or more"  will definitely spawn more Plague Zombies 1D4+1 for every time a Thunder type spell is cast with the before mentioned descriptor.