In battle
When a Pokémon with this Ability is hit by a move that makes contact, the Ability of the attacking Pokémon will become Mummy. Besides spreading Mummy in battle, the Ability does not have any other effect.
If a Pokémon with this Ability is hit by a multi-strike move that makes contact (such as Fury Swipes), the attacking Pokémon's Ability will become Mummy after the first strike.
If a Pokémon with Rock Head uses a recoil contact move on a Pokémon with Mummy, its Ability will become Mummy, then subsequently it will take recoil damage. If a Pokémon with Parental Bond uses a contact move on a Pokémon with Mummy, the Pokémon's Ability will become Mummy after the first strike, then the current move will continue to strike a second time regardless.
Multitype, Stance Change, and Mummy itself are not affected by Mummy.
Outside of battle
Mummy has no effect outside of battle.