Well, the different types of depression aren't necessarily distinct. Life events, both emotional and physical, can trigger biochemical changes that cause depression and don't always resolve when the physical or emotional event is over. For example, it's been proven that any serious physical trauma, like a surgery, a car accident, or childbirth, can alter a person's thyroid and/or adrenal hormones, and in some cases, the changes lasts for months, years or forever. And of course, those hormones play a big part in a person's mental health. This is probably why there's so much confusion about treating depression--the line between physical and emotional causes is indeed blurry.