Nicholas A. Christakis, MD, PhD, is a professor at Harvard University with joint appointments in the Departments of Health Care Policy, Sociology, and Medicine, and in 2009 was named one of Time magazine's 100 most influential people in the world. James H. Fowler, PhD, is an associate professor at the University of California, San Diego, in the Department of Political Science and The Center for Wireless and Population Health Systems, and was named one of the "most inspiring scientists" by the San Diego Science Festival. Christakis and Fowler's research has appeared on The Oprah Winfrey Show, Good Morning America, Today, and The Colbert Report, and on the front pages of the New York Times, Washington Post, and USA Today.