I watched black-bullet and read the light novel quite some time ago. But just recently watched it again because I got bored.
Something got to my mind all of a sudden after episode 4, Why is the Tricycle the catalyst for a stage 5 Gastrea?
I looked around on the wiki etc, but couldn't really find it. But I have my speculations, like if the human soul or something remains a little and this is used as a triggering factor for a lot of emotions that it would form an stage 5 or something.
Nevertheless, I'm really interested if I missed it and there is an actual mention of this or other's speculations.