Depression (major depressive disorder) is a common and serious medical illness that negatively affects how you feel, the way you think and how you act. Fortunately, it is also treatable. Depression causes feelings of sadness and/or a loss of interest in activities you once enjoyed. It can lead to a variety of emotional and physical problems and can decrease your ability to function at work and at home.

Depression symptoms can vary from mild to severe and can include:

feeling sad or having a depressed mood; loss of interest or pleasure in activities once enjoyed; changes in appetite-weight loss or gain unrelated to dieting; trouble sleeping or sleeping too much; loss of energy or increased fatigue; increase in purposeless physical activity (e.g., inability to sit still, pacing, handwringing) or slowed movements or speech (these actions much be severe enough to be observable by others); feeling worthless or guilty; difficulty thinking concentrating or making decisions; thoughts of death or suicide. 

Symptoms must last at least two weeks and must represent a change in your previous level of functioning for a diagnosis of depression.