"Feelings are contagious. “Each happy friend a person has increases that person’s probability of being happy by 9 percent and each unhappy friend decreases it by 7 percent,” says Nicholas A. Christakis, a co-author of “Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives.” Males with depressed roommates may end up feeling a bit blue themselves, according to Daniel Eisenberg, an assistant professor of public health at the University of Michigan who recently led a survey of 1,600 freshmen at two universities — a public one in the Midwest and a private one in the Northeast — on the issue. He found no such carryover for female students. This mood contagion seems to occur when the student keeps his feelings bottled up, Dr. Eisenberg says. And it’s only a mild case; roommates typically don’t develop their friends’ more serious conditions. “It’s not like you catch a mental-health cold,” he says. “People are resilient. They have a lot of coping strategies.”"