User blog comment:San&Ashitaka/TITAN THEORIES/@comment-17563801-20131013213302

i say its a virus, a genetically modified virus. theres millions of titans in the snk universe no way they were all man-made, it has to be an epidemic that turned them all into titans. and somehow the people of the walls are immune to it, so maybe that liquid in needle that grisha injected into eren was a "de-immune-lizer" (lol i dont know if thats even a proper word).