"Got splashed with the same goo that turned me into this. But he got bit by a rat before that, so I guess that's where the mix came from."

Beyond that, Leo doesn't understand it at all. Donnie had done plenty of opining on how it worked, and Leo had tuned every bit of it out. All he knows is that there was magic in the science goop.

"I guess the whole lab exploded while he was trying to escape. ...Huh, and then we did the same thing thirteen years later! Heh, no wonder Barry didn't like us." He laughs, like making mortal enemies with a guy who wanted to kill all of humanity is funny.