For two decades, scientists and doctors have relied on blood types to categorize patients. Depending on whether one is blood type A, B, AB, or O, doctors could alter their treatment to increase their chances of a successful procedure. But there's a new way for people to be categorized medically — gut bacteria. New research shows that there are three distinct ecosystems in people's guts that could have direct effects of people's heath. We talk with Carl Zimmer, science reporter for our partner The New York Times, who reported on this story in yesterday's paper.