Depression is a serious medical illness that negatively affects how you feel, the way you think and how you act. It can occur at any time, but on average, first appears during the late teens to mid-20s. However, this is a treatable condition. 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. Symptoms must last at least two weeks and must represent a change in your previous level of functioning. Also, medical conditions (e.g., thyroid problems, a brain tumor or vitamin deficiency) can mimic symptoms of depression so it is important to rule out general medical causes.