According to the book of Enoch the pre-flood world of earth was inhabited by Nephilim giants. These were the offspring of women who were impregnated by the fallen angels. These giants were so evil and so destructive to mankind that God brought a flood to to destroy them. Once dead the spirits of the Nephilim now roamed the earth as demons seeking to inhabit flesh once again. Without a fleshly host to posses they suffer discomfort.