Horde. Any time a loud noise is made (louder than an indoor voice) it attracts 1d4 more zombies. A really loud noise (e.g. thaumaturgy) attracts 2d4 more zombies.

Tenacious Z Whenever the zombie is reduced to 0 hit points by damage that is neither radiant nor the result of a critical hit, roll a d4, on a roll of 3 or 4 the zombie regains 1 hp and immediately uses it's reaction to stand up again. If dismembered it is the largest portion of the zombie's body which remains alive. 

Bite and infect. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) piercing damage.

Any time a living creature is hit it needs to make a DC 10 CON saving throw to try to resist the infection, on a fail the creature is infected with the Zombie Virus.

Once infected a living creature loses 1 Max HP per day until they reach zero and become a Humanoid Zombie with 22 HP. If they die or fall to 0 HP at any time while infected they instantly come back as a Humanoid Zombie with 22 HP.

Only removing the part of the body where the infection occurred within 24 hours or a Greater Restoration spell can prevent the loss of HP and becoming a Humanoid Zombie with 22 HP.