The image of a depressed caveman avoiding the hunt seems funny, but it might not be far from the truth. A group of psychiatrists now thinks that depression helped early humans survive infections. This could also explain why genes linked to depression continue to exist today. From the standpoint of evolution, genes that help people survive, and have children, tend to be passed onto offspring. For early humans, the microscopic world of bacteria and viruses was more deadly than saber-toothed tigers. Over time, the body developed ways to fight infections, such as fever, fatigue, social withdrawal and decreased eating. The psychiatrists noticed that these activities are also present during depression. In fact, previous studies have linked depression with increased activity in the immune system, which occurs even when a depressed person isn’t fighting an infection. Rather than being an illness, depression is a sign that the body is preparing for infection or injury. Stress, in fact, is a trigger for major depression. The problem, though, is that during depression the immune system is activated even without the presence of an infection. The researchers hope that this will lead to future research, such as predicting how people will respond to depression treatments, or using medications for auto-immune diseases to treat depression. In the meantime, if the depressed caveman doesn’t have the cold or flu, he should rejoin the hunt. Maintaining relationships and social activities during depression is important for coping.