When a story, especially something as long as a novel begins to take shape in a writer's mind, characters often click into place. Or they evolve in a painstaking process. Occasionally, one darts into view in stealing glances or a flash of feathers. This happened to me in the summer of 2005 when I began to construct a novel around a modern-day flu pandemic. I was looking for a pivotal character in the form of an animal, particularly a bird that could carry the mystery of the story and take it beyond a human catastrophe. For a while, I considered the Ivory Bill Woodpecker, an "extinct" bird making headlines at the time. But this species had current widespread media coverage. I was looking for a ghost-a real one. That's when I discovered the passenger pigeon, one of America's saddest extinctions, and the likely carrier of a controversial anti-virus in my novel.