'Wicked Plants' author Amy Stewart on the dangers of ricin-tainted letters

Reports that someone may have sent ricin-laced letters to President Obama and New York Mayor Michael Bloomberg left us wondering just how dangerous ricin really is. In Book World, we turn to Amy Stewart with our poison- and death-threat-related questions. She's the author of six books, including "Wicked Plants: The Weed That Killed Lincoln's Mother and Other Botanical Atrocities."