A recent study that has been getting a lot of press coverage asserted that happiness may be contagious. This raises the question about whether depression may have a contagious element. The authors used social network theory to show that the probability of being happy was greater when other around you are happy - and found weaker effects for unhappiness. The idea of emotional contagion is a very old one, but its relevance to depressive disorders has not been confirmed. The outbreak of sports riots is another circumstance in which emotional contagion may play an important role, see here.