If a physical move that is effective against somebody with mummy, mummy will spread to the attacker. Ill give the example of a ghost type.
A Eevee uses tackle on a Yamask. It was not effective. Yamask used Will-o-wisp on the Eevee. The Eevee was burned. Eevee used Shadow Ball on the Yamask. Its super effective. Yamask used Will-o-wisp. Its not effective. Eevee used Bite on the Yamask. It was super effective. Eevee gained the Mummy ability.
As you would see the Eevee didnt gain the Mummy ability until it attack with Bite, which is was a physical move and it was effective. When it used tackle it didnt effect the Yamask so Eevee didnt get the Mummy ability. Also status moves wont spread it.