Depression, though one of the most common forms of mental illness in the world, is still vastly misunderstood. In part, this is because it’s difficult to differentiate between the intermittent episodes of sadness everyone experiences at some point and the deeper, more serious feelings of sadness that arise in clinical depression — something only those who experience it can really comprehend.

Researchers aren’t always sure what causes depressive disorders, but genetics, abuse, medications, and major life events all have the potential to contribute to an episode of depression. That said, the illness also appears in many forms, each with its own symptoms and different levels of severity, which can make it even more difficult for doctors to study and patients to deal with.

Click “View Slideshow” to learn about six different variations of depression, from the most common to the rarest forms.