In Fallout, becoming a ghoul is a result of exposure to high levels of radiation combined with an unknown genetic factor. While most people will die a horrible death as a result of such exposure, a fraction will instead mutate into ghouls, who are not harmed, but sustained by regeneration. The process is gradual and may include loss of skin and some soft tissue over time, lesions, cancers, which do not kill the ghoul, but accumulate, twisting their appearance.