Depression is commonly referred to as the "common cold" of mental illness. This metaphor refers to high prevalence of depression, and given depression's especially high prevalence among people seeking help from primary care physicians (10% or more of primary care patients are depressed), it makes sense that it would look quite mundane to doctors. But the metaphor is in many ways misleading: you don't catch depression from someone else and you don't recover from a major depressive episode after a couple days of rest. The metaphor trivializes a mental illness with profound and concrete consequences -- loss of work productivity, severe unhappiness, suicidal thoughts and actions -- for its sufferers.
If not the common cold, is there any good parallel to depression in the realm of physical illnesses? Researcher and psychotherapist Jon Adler has told me that he compares depression to diabetes. The diabetes metaphor has an elegance that the common cold metaphor does not. Both diabetes and depression are diseases of modern life. Their courses are chronic, and they need to be managed behaviorally. Diabetes is an especially good metaphor to use with clients in the course of behavior therapy because diabetes management requires a lot of self-monitoring and self-care, just as depression treatment does. The metaphor can encourage clients to do the tasks (e.g., scheduling doing pleasurable activities) that behavior therapy entails.
In the end, though, metaphors are always flawed. What all of us in the mental health community would really like would be to have depression recognized more widely as a legitimate illness in its own right -- high in prevalence as well as high in severity -- so that we can rely less on other disease metaphors.