User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-29874610-20170204151505/@comment-1776824-20170204225518

Inert = inactive; not in effect; sleeping; not working; etc. Natsu has a human body, but he is an Etherious by definition only. He only started transforming after his battle with Zeref. I've already explained it once. He is physiologically a human being. It's not rocket science.