Minecraft wiki entry for Zombie, associated text: In Java Edition, on Hard difficulty, zombies can spawn additional zombies to "help" when damaged. Each zombie has a "likeliness to call reinforcements" statistic ranging from 0–10%, and "leader" zombies (0–5% depending on regional difficulty) get a bonus of 50–75 percentage points to the stat. When the zombie is damaged by an entity or is damaged while targeting an entity, up to 50 attempts are made to randomly choose a spawn location (0 or ±7–40 blocks away in all three axes) that is above a block with a solid top surface, has light level 9 or lower, has no players within 7 blocks, and has no colliding entities or blocks at which to spawn the reinforcement. Both the damaged zombie and the new zombie has a 5 percentage point penalty to their "likeliness to call reinforcement" stat, preventing infinite zombies from spawning this way. Zombie reinforcements can spawn even in locations where naturally spawned zombies can't spawn, like mushroom biomes, The End, and Nether biomes.[4] However they still can not spawn on blocks where other items like redstone dust or pressure plates are placed.