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.

Infection. When the zombie successfully hits a creature with a Bite attack, that creature must make a Constitution saving throw with a DC of 12, becoming infected on a failed save.

Immediately upon infection, and once every hour afterwards, the creature will take necrotic damage. This damage lowers the creature's hit point maximum equal to the amount of damage dealt.

This effect lasts until the creature dies or if it is cured. If the creature dies, it's corpse will reanimate as a zombie. If the infection is cured, the creature will regain it's original hit point maximum. 

The infection can be cured with the Remove Curse or Greater Restoration spells. It can also be cured by removing the infected limb before it has dealt 12 damage. Creatures that are immune to disease cannot become infected.

Infection Tick. 3 (1d6) necrotic damage.

Scratch. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) slashing damage.

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) necrotic damage.