User blog comment:B214/Question and Answers/@comment-79.32.175.82-20130511112258/@comment-9297146-20130511143421

After the curse was done with Issei what was left was probably either a desecated husk of his body or like a strand of his hair. Either way he's now almost like the son of Ophis and Great Red using his own DNA as an intermediary which his why his life force keep switching  back and forth between 0 and infinite. It's a supernatural process. It doesn't need to make a whole lot of sense.