Depression affects 47.8 million Americans in 2025 — a historic high — yet the average person waits 11 years between symptoms starting and getting treatment. This complete guide covers the 9 DSM-5 diagnostic criteria that distinguish depression from ordinary sadness, all 7 depressive disorder types, the biological causes including neuroinflammation and cortisol damage, the full antidepressant comparison table, and treatment options for treatment-resistant depression including