Superorganisms are one of my favorite concepts.
A superorganism is an organism consisting of many organisms. For example, an ant colony can be considered a superorganism because its members are highly specialized and somewhat simple and yet the colony generates sophisticated behavior as a result of the interactions of its members.
Following these lines, our own society can be considered a superorganism. In his book The Lucifer Principle, Howard Bloom postulates a fascinating theory related to superorganisms. He believes that long-term depression is a person's built-in mechanism for self-destruction, similar to the programmed cell death that occurs when a cell is given certain signals from its neighbors, stress, or DNA damage.
The idea is that if a person is shunned by their friends and relatives, can't hold a job, or otherwise is rejected by society, then they are a drain on the superorganism and should therefore self-destruct. So depression kicks in and weakens their immune system, thereby increasing the chance of infection and death. It's a slow process, so fortunately there's plenty of time to change and recover!
Overall, the theory makes sense to me and has expanded my understanding of emotion.
Update: Bloom refers only to depression that is caused by social isolation, not to depression that results from, say, a chemical imbalance. Apparently social isolation and/or uncontrollable punishment consistently induce depression in laboratory animals.