Depression (major depressive disorder) is a common and serious medical illness that negatively affects emotions and behaviors. Depression can cause feelings of sadness and a loss of interest in activities once enjoyed. It can lead to a variety of emotional and physical problems and can decrease one’s ability to function at work and at home. Depression symptoms can vary from mild to severe and can include issues such as feeling sad or having a depressed mood, changes in appetite, trouble sleeping, feeling worthless or guilty, loss of interest or pleasure, and thoughts of death and suicide. Symptoms must last at least two weeks and must represent a change in previous level of functioning for a diagnosis of depression.