As my last keystrokes about depression here on Hacker News pointed out, there isn't just one disease known as depression. Depression is a symptom pattern (prolonged low mood contrary to the patient's current life experience) found often in the broad category of illnesses known as mood disorders. Behavior genetic studies of whole family lineages, genome-wide association studies, and drug intervention studies have all shown that there are a variety of biological or psychological causes for mood disorders, and not all mood disorders are the same as all other mood disorders. I know a LOT of people of various ages who have these problems, so I have been prompted for more than two decades to dig into the serious medical literature[1] on this topic. (I am not a doctor, but I've discussed mood disorders with plenty of doctors and patients.) I've seen people who tried to self-medicate with street drugs end up with psychotic symptoms and prolonged unemployment, and I've seen people with standard medical treatment supervised by physicians thrive and enjoy well off family life. The best current treatment for depression is medically supervised medication combined with professionally administered talk therapy.[2]
The human mood system can go awry both by mood being too elevated (hypomania or mania) and by it being too low (depression), with depression being the more common symptom pattern. But plenty of people have bipolar mood disorders, with various mood patterns over time, and bipolar mood disorders are tricky to treat, because some treatments that lift mood simply move patients from depression into mania. And depression doesn't always look like being inactive, down, and blue, but sometimes looks like being very irritable (this is the classic sign of depression in teenage boys--extreme irritability--and often in adults too). Physicians use patient mood-self-rating scales (which have been carefully validated over the years for monitoring treatment)[3] as a reality check on their clinical impression of how patients are doing.
As the blog post kindly submitted here points out, a patient's mood disorder influences the patient's whole family. The more other family members know about depression, the better. Encouraging words (NO, not just "cheer up") are important to help the patient reframe thought patterns and aid professional cognitive talk therapy. Care in sleep schedules and eating and exercise patterns is also important. People can become much more healthy than they ever imagined possible even after years of untreated mood disorders, but it is often a whole-family effort that brings about the best results.
[1] http://www.amazon.com/Manic-Depressive-Illness-Disorders-Rec...
[2] Combination psychotherapy and antidepressant medication treatment for depression: for whom, when, and how. Craighead WE1, Dunlop BW.
Annu Rev Psychol. 2014;65:267-300. doi: 10.1146/annurev.psych.121208.131653. Epub 2013 Sep 13.