Zombies

Naturally
In the Overworld, zombies spawn in groups of 4 at a light level of 7 or less and constitute about 13% of mobs. In desert biomes, all zombies exposed to the sky have an 80% chance to be replaced by husks, a zombie variant. Zombies that are not husks have a 5% chance to spawn as a zombie villager while all zombie variants also have a 5% chance to spawn as a baby zombie type. Baby zombies have an additional 5% chance of spawning as a chicken jockey.

A husk that drowns in water drops any equipment being worn or held, and converts to a zombie, which then converts to a drowned if its head remains submerged.

Sieges
Main article: Zombie siege

If a player is in a village with at least 10 doors and 20 villagers at midnight, up to 20 zombies may be spawned near the edge of the village regardless of light level or transparent blocks. It can happen in any biome, even if it is Mushroom Island. Zombies spawned as a part of a siege are always be normal zombies; zombie villagers or husks never spawn as a part of a siege.

Monster spawners
Zombies can spawn from monster spawners inside dungeons.

Baby zombies
A baby zombie.

An armored baby zombie riding a chicken.

Baby zombies make up 5% of zombie spawns. They behave similar to regular zombies, with the following oddities: A baby zombie riding a chicken is known as a chicken jockey. In Bedrock and Legacy Console editions, a baby zombie can ride:
 * They are 30% faster than the normal zombies, yet they have the same HP as normal zombies. This makes the baby zombies more dangerous than their bigger counterparts.
 * The noises they make are higher-pitched than normal zombie sound effects.
 * If a baby zombie is riding a mob that is also rideable by a player, that mob becomes un-rideable by any player.
 * A baby zombie has a 15% chance to become a jockey when it tries attacking a villager, player, or golem.
 * Baby zombie villagers turn into baby villagers if cured.
 * Worn armor shrinks to fit their body size.
 * They are able to fit through 1×1 block gaps.
 * They give 12 experience points when killed by the player or a tamed wolf instead of 5.
 * Unlike all other baby mobs in the game, they remain babies indefinitely, and never become fully-grown zombies.
 * They have a decreased hitbox size.