User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-29874610-20170204151505/@comment-1776824-20170204214208

Demon Seed being present but inert until Zeref pierced the book literally explains everything. It's not a difficult concept. Natsu is an Etherious with the body of a human being. Why would anything demon-related remotely affect his completely normal body?