Ruslan M. Medzhitov loves scientific puzzles. And this penchant has led him to tackle some of the big questions of modern biology. At Yale University, where he is a professor of immunology and a Howard Hughes Medical Institute investigator, Dr. Medzhitov, 45, helped make key discoveries in the workings of vertebrates’ immune systems. We spoke about them at his home in Guilford, Conn.; again in New York City; and finally by telephone. An edited and condensed version of the sessions follows. Let’s talk about the paper by the immunologist Charles A. Janeway Jr. that changed your life. When did you...