Senses. Zombies hunt by hearing first, then by smell. Their eyesight is poor, and worst at night.
Grab, then bite. A zombie must grab (grapple) a creature before it can bite. A bite that deals damage infects.
Killing them. A zombie falls for good when its brain is destroyed. A critical hit counts as a head hit. Fire also ends them.
Keep Coming and Relentless. When a zombie drops to 0 HP, it may stay up at 1 HP. The tough zombies have Keep Coming. The weak ones (Zombie, Screamer, Husk and Crawler) have Relentless, which only works the first time. Neither works against a critical hit or fire. The full rule is on each stat block.
Animals. Zombies are only people. There are no zombie animals.
Stats: CR 0, 1 to 8 HP. A zombie that lost its legs. It drags itself along the ground with its arms. It hides under cars, in ditches and in tall grass, and grabs the ankles of anyone who walks past. How to handle it. Look under cars and in low places before you walk past. It is very slow, so step back out of reach and crush its head.
Stats: CR 0, 1 to 8 HP. A zombie that is 20 years old or more and has dried out. Its skin is brown and tight over the bones. It is brittle and breaks apart under heavy blows. It is also silent, because it has no voice left. Husks are common in deserts and hot dry places. How to handle it. Use a club, bat or hammer. Blunt weapons break it apart. Keep watch in every direction, because you will not hear it coming.
Stats: CR 1/8, 1 to 8 HP. The common dead. Grey skin, black veins on the neck and arms, cloudy white eyes, torn old clothes. It is slow, but it never stops once it hears you. Most zombies in any dead city are this kind. How to handle it. One is easy. Ten are deadly. Fight them one at a time in a doorway or narrow hall, and aim for the head.
Stats: CR 1/8, 1 to 8 HP. A zombie with a torn throat. When it sees or hears the living, it lets out a high shriek. The shriek calls every zombie within 300 feet. How to handle it. Kill it first, and kill it quietly. A bow or crossbow from range is best. If it screams, leave the area.
Stats: CR 1, 30 HP. A person who turned while wearing riot gear or body armor. The armor and helmet are still on it. Its helmet stops critical head hits, so it is hard to put down fast. How to handle it. Knock it down and hit it again and again. Fire still ends it. Its armor and helmet can be looted after it falls.
Stats: CR 2, 45 HP. A zombie that was fit and strong in life. It sprints, jumps fences and leaps onto its target. It is much faster than a common zombie. How to handle it. You cannot outrun it on open ground. Get behind a door, climb, or stand your ground in a narrow space and fight.
Stats: CR 3, 68 HP. A zombie swollen with gas and rotten fluid. It is huge, slow and soft. When it dies it bursts and sprays infected fluid. Anyone caught in the spray makes the blood and bile save (Constitution save DC 13). How to handle it. Kill it from range, never up close. Wear goggles or a gas mask. Stay out of the puddle it leaves.
Stats: CR 3, 52 HP. A quiet zombie that climbs walls and ceilings. It waits in dark places, such as stairwells, attics and air shafts, and drops on its target. How to handle it. Carry light. Look up when you enter a dark room. Move in pairs and watch each other's backs.
Stats: CR 4, 60 HP. A zombie that spits infected bile at the face from 30 feet away. A target hit in the face makes the blood and bile save (Constitution save DC 13). How to handle it. Wear goggles or a gas mask for advantage on the save. Use cover to close the distance, or shoot it from beyond 30 feet.
Stats: CR 4, 85 HP. A zombie with a large, heavy body. It breaks through doors and barricades. A locked door does not stop it. How to handle it. Do not count on a barricade. Keep moving, hit it from several sides, and use fire if you have it.
Stats: CR 5, 100 HP. The worst single zombie. It is in a frenzy all the time and makes three attacks each turn. It does not stop until it is dead. How to handle it. Avoid it if you can. If you must fight, use traps, fire and high ground, and shoot from where it cannot reach you.
Stats: CR 2, 60 HP. A swarm of about 8 zombies. A small group of zombies that moves together as one threat. In a dead city, a Loud noise brings a Zombie Pack in 1d4 rounds. How to handle it. Hold a doorway so only one or two can reach you at a time. Fire works well on a pack. If you cannot hold, run.
Stats: CR 5, 100 HP. A swarm of about 30 zombies. A big group of zombies. In a dead city, an Alarm noise brings a Zombie Horde in 1d4 rounds, and another every minute after. In a dead city a horde regains 10 HP each turn, because more zombies keep joining it. How to handle it. You do not beat a horde. You get away from it. Run for water, a boat, a roof or a strong wall. Throw a noise to one side to pull it away from you.