[SB]When exposed to the mako capsule, Cloud lost his mind and needed something to grasp onto - when Zack died, his personality was that 'thing' to grasp and somehow he became almost the same person, even adopting his memories.
When was Cloud exposed to Mako? And why did he end up being so strong if he was only a lowly guard at first?

This was probably in the game, but I only played through it once and that was a few months ago.