While there are dozens of types of mental illnesses, central to all of them are thoughts, feelings and behaviors that get in the way of living a normal life. It is the extent to which they interfere with daily living, and the length of time this interference exists, that separates a mental illness from everyday blues or worries.

Everyone feels anxious or sad at times. Anxiety is a natural response to a stressful situation, especially one that we anticipate might end badly. Sadness is a natural response to losing something that we care about. What turns each into a mental illness is the degree of intensity, the duration, and often, the lack of connection with what is actually occurring at the moment.

People with anxiety disorders have fearful thoughts, feelings and at times physical symptoms even when nothing serious is threatening them. People with depressive disorders often have painful feelings of sadness and guilt and persistently negative thoughts even if their lives are otherwise going quite well. On the opposite end of the spectrum, people with bipolar disorder who are in a manic phase may feel euphoric and powerful even when their lives are crumbling around their ears.

Depression and anxiety can vary a great deal in their severity and duration. For some people, depression and anxiety can be a temporary setback, for others it is a lifelong struggle. Some people have milder forms of the illnesses, while others suffer crippling and even life-threatening forms of depression and anxiety.

Bipolar disorder is generally believed to be a lifelong disease. Some people with bipolar disorder cycle rapidly between depressions and manic episodes. Others have longer, slower cycles. In general, a manic phase is almost always followed by a depression. The severity of the depressions and manias can also vary, both between two people, and within a single person over the course of his or her life.

While some people with depression or anxiety disorders can be treated with short-term therapy or medication—or both—people with bipolar disorder generally need to be on continuous lifelong medication

Last Reviewed: June 14, 2010