Depression is a common and serious medical illness that negatively affects how you feel, the way you think and handle daily activity. Depression affects one in 15 adults in any given year. And one in six people will experience depression at some time in their life. Current research suggests that depression is caused by a combination of genetic, biological, environmental, and psychological factors. Depression can happen at any age. Major depression can cause a variety of symptoms. Some affect your mood, and others affect your body. Symptoms may also be ongoing or come and go. Common symptoms of depression are following: feeling sad or having a depressed mood, loss of interest or pleasure in activities once enjoyed, changes in appetite, feeling worthless or guilty, trouble sleeping or sleeping too much, loss of energy or increased fatigue, slowed movements and speech (actions observable by others), difficulty thinking, concentrating or making decisions, thoughts of death or suicide. Symptoms must last at least two weeks for a diagnosis of depression. For many people with depression, symptoms usually are severe enough to cause problems in day-to-day activities, such as work, school, social activities or relationships with others. Although depression is among the most treatable of mental disorders. Between 80 percent and 90 percent of people with depression eventually respond well to treatment. Almost all patients gain some relief from their symptoms.