Theory

I'm in episode 14 of Attack on Titan. My guess is that the titans were created by humanity. Perhaps created to be a shared threat for humanity to unite behind. It may have been a virus released into the population that transformed a number of people and killed even more. The survivors were immune. That would explain how so few humans survived. Books from before the titans still exist, so chances are good that Eren's dad had seen some. (Perhaps through a benovolent secret organization.) What was learned in the book helped to recreate parts of the knowledge and gave Eren a form of the virus that allows him to become a titan and remain human at the same time.

I guess I'll see as I continue to watch.