Depression can creep up gradually, which makes it harder to identify than a sudden illness. A bad day turns into a rut and you start skipping work, school, or social occasions. One type, called dysthymia, can last for years as a chronic, low-level illness – a malaise that silently undermines your career and relationships. Or depression can become a severe, disabling condition. With treatment, many feel substantial relief in 4-6 weeks.