Five days ago, a homeless man on a Times Square subway platform died in agony as startled commuters looked on. Yesterday, a teenager at a private girls' school on the Upper East Side started having violent, uncontrollable spasms. Within minutes, she too was dead. Dr. Alice Austen, a medical pathologist at the Centers for Disease Control in Atlanta, is the only one who knows the two deaths are connected. What she fears is that they represent the beginning of a horrific national catastrophe. . . .