MDD stands for Major Depressive Disorder or depression as most of us know it. Depression causes symptoms including sleep and appetite disturbance, loss of pleasure in activities, poor motivation, poor energy, irritability, isolation, feeling slowed down or sluggish, and potentially thoughts of death or suicide. MDD may occur as a single episode, or for some the symptoms may be recurrent. Depression may happen with or without a specific cause. For instance, depression can follow trauma, life changes, or injuries to the brain. Additionally, MDD tends to have a genetic link.