Anonymous5: It's smallpox, actually. It's what Sadako kills people with: a curse that inflicts a modified smallpox virus, creating a tumor that results in a heart attack in seven days (unless they help propagate the curse, in which case she lets them live).
Anonymous6: Or if they're ovulating, in which case she impregnates them with a clone of herself, possesses them, then traps them somewhere to die without being discovered while the newly born clone slips away.
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Prepare. Your. Dick.