Everyone feels sad or low sometimes, but these feelings usually pass with a little time. Depression (also called major depressive disorder or clinical depression) is different. It can cause severe symptoms that affect how you feel, think, and handle daily activities, such as sleeping, eating, or working. It is an illness that can affect anyone— regardless of age, race, income, culture, or education. Research suggests that genetic, biological, environmental, and psychological factors play a role in depression.