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.

Infectious Bite/Claw. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) piercing damage. On a hit the target must make a DC10 Con, Str, or Dex saving throw (The target's choice) or be infected. On a failure the target immediately loses one max hp and loses one max hp per long rest. If the creature's max hp drops to 0 or if the creature dies, they rise as a zombie with full 22hp. A Greater Restoration spell removes the infection and restores the creature's max hp.