User blog comment:IamJakuhoRaikoben/Chapter 501: Mari and Randi, Review./@comment-5303512-20160913202233/@comment-5303512-20160913203846

And after that, did any of those people get cancer from it? No? Didn't think so.

This is the case where Zeref shot a hole through the book of END, and in turn it effected Natsu the same way. But we find out where the tumor is located at that same place where he felt that pain. There is obviously more to this than What it seems.