User blog comment:Eddienam/Chapter 53 mystery/@comment-76.31.149.155-20140109032232/@comment-24339754-20140112030757

I agree, Im pretty sure the injection was the reason for Titan shifting. But I don't that was the only way to become a titan shifter. Because if you think about the Ymir case, you can kind of see that by eating another titan shifter, you may recieve that power. But on the other hand, Ymir says she lived as a titan for 60 years, so i can kinda see how Child of the titan might be involved in it.