The supporters of presidential candidate John McCain aggressively pooh-pooh concerns about his age and health history. That which hasn't killed him, they argue, has made him stronger. But a study of past American leaders reveals that those who think McCain's long history of toughness makes him invincible had better think again. Of the eight presidents who have died in office, four were killed by assassins. Youth and a clean bill of health remain no match for bullets. The running mates of even young and vigorous presidents should be completely qualified to take over all presidential duties in a heartbeat. But older candidates with a history of health concerns and their running mates require special scrutiny.