Depression is a common and serious medical illness that causes prolonged feelings of sadness and loss of interest in activities. It can lead to emotional and physical problems that decrease ability to function. Signs include changes in appetite, sleep, energy levels, concentration, feelings of worthlessness, and thoughts of suicide. Depression is caused by a combination of genetic, biological, environmental, and psychological factors. It can affect people of all ages but often begins during the teenage years or early adulthood. Treatment options include psychotherapy, medication, light therapy, exercise, and social support.