In April of 1987, a woman in Westchester County gave birth to a little girl. Among the traits she passed on to her baby were curly black hair and a wide, toothy smile, with an upper lip shaped like the double arches of a child's drawing of a bird in flight.She also passed on a deadly disease that had been named only a few years earlier: AIDS. The mother, a prostitute, had probably contracted it from a dirty heroin needle. More than 4,000 Americans...