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Natalia Alianovna "Natasha" Romanoff (Russian: Наталья Альяновна "Наташа" Романов), also known as Black Widow, is a trained assassin, former S.H.I.E.L.D. agent, and a member of the Avengers.
Zombie Physiology: Black Widow was infected with an encephalopathic virus that overloaded her brain's limbic system, turning her into a zombie.
Infection: Like any zombies, Black Widow is capable of spreading encephalopathic virus to others by biting or scratching which would turn them into zombies.