"What do Vandal Savage, Ra's Al Ghul and the Joker have in common? A lot more than past DC Comics history would have you believe. According to Batman #38, Vandal Savage acquired his immortality from the radiation of a mysterious meteorite that fell to Earth in the prehistoric era. Ra's Al Ghul apparently bathes in a "corrupted version" of the chemical/radiation. And the Joker, as told in Snyder's Batman #38 also encountered the meteorite before Gotham was ever born. That's right, the Joker is immortal. It's a clever retcon to explain how the Joker bounces back time after time from all the punishment Batman dishes out. As explained in Batman #38, the chemical only activates in the Joker at moments of extreme physical injury."
http://www.comicbookmovie.com/fansites/GraphicCity/news/?a=114664
http://www.comicbookmovie.com/fansites/GraphicCity/news/?a=114664