Short answers, pulled from the story.
Major depressive disorder is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. The term was introduced by a group of US clinicians in the mid-1970s and formally adopted by the American Psychiatric Association in the DSM-III in 1980.
Major depressive disorder is believed to result from a combination of genetic, environmental, and psychological factors, with genetic factors accounting for nearly 40% of the variation in risk. Environmental contributors include adverse childhood experiences, living alone, air pollution, and major life changes. Biological theories involve monoamine neurotransmitter systems, HPA-axis dysfunction, immune system abnormalities including elevated pro-inflammatory cytokines, and gut-brain axis dysfunction.
Diagnosis is based on reported experiences, observations from family or friends, and a mental state examination; there is no laboratory test. Under DSM-5, at least five of nine specified symptoms must occur most of the time for more than two weeks, impairing functioning, with at least one symptom being either depressed mood or loss of interest or pleasure. Blood tests may be performed to rule out physical conditions that can mimic depressive symptoms.
The most common and effective treatments are psychotherapy, antidepressant medication, and electroconvulsive therapy, with a combination approach most effective for treatment-resistant cases. SSRIs are the primary medications prescribed due to mild side effects; response rates to a first antidepressant range from 50% to 75%. Cognitive behavioral therapy is the most extensively studied psychotherapy and can perform as well as antidepressants in people with major depression.
About 2% to 8% of adults with major depression die by suicide, and approximately 50% of all people who die by suicide have a mood disorder such as major depression. People with the disorder also face a 1.5 to 2-fold increased risk of cardiovascular disease independent of other known risk factors. Lithium augmentation reduces the risk of suicide by 87% in people with depression or bipolar disorder.
Major depressive disorder affected approximately 163 million people in 2017, about 2% of the global population. In the United States, 8.4% of adults have at least one episode within a given year, with rates highest among those aged 18 to 25 at 17%. Despite its prevalence, only about 35% of those suffering from the disorder worldwide seek professional help.