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

Minion. Minion Creatures only have one hit point. A missed attack never damages a minion. When a minion is subjected to an effect that allows it to make a saving throw to take half damage, it instead takes no damage if it succeeds on the saving throw. Minions always deal average damage, and never have legendary or lair actions. 

The Horde. Zombie Minions accumulate in areas and gather as groups, any area where these Zombie Minions congregate has the following properties:

Swarm. While these zombies lack higher brain function, they respond violently to loud noises. When a loud noise happens 2 (1d4) additional Zombie Minions, if any are within 100’, will dash towards the location of the sound and attack any living thing that might have caused it. A swarm contains 50 (5d20) Zombie Minions. Subtract the 2 (1d4) from this total.

Slam. 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 the target's choice of a DC10 Strength, Dexterity, or Constitution saving throw or be infected. The infected targets hit point maximum decreases by 1 for every 24 hours that elapse. If the target dies, is knocked unconscious, or if the infection reduces the target's hit point maximum to 0, it becomes a Zombie with 22 hit points. The infection lasts until removed by the Greater Restoration spell or the infected limb is removed.