View full lesson: http://ed.ted.com/lessons/what-is-depression-helen-m-farrell Depression is the leading cause of disability in the world; in the United States, close to ten percent of adults struggle with the disease. But because it's a mental illness, it can be a lot harder to understand than, say, high cholesterol. Helen M. Farrell examines the symptoms and treatments of depression, and gives some tips for how you might help a friend who is suffering. Lesson by Helen M. Farrell, animation by Artrake Studio.