Depression is a mental illness that affects a person's body, feelings, thoughts, and behavior. Millions of people in the United States suffer from depression, and about twice as many women as men suffer from it. Everyone experiences periods of sadness now and then. However, if these feelings last more than a couple of weeks or interfere with daily life, a person may be suffering from depression. Depression involves a set of symptoms that can last for months and sometimes years. It is not a sign of personal weakness or a condition that can be willed or wished away. People with depression cannot merely "pull themselves together" and get better. The causes of depression are not always known. Research shows that it may be inherited and that an uneven balance of naturally occurring mood-influencing chemicals in the brain can play a role in its development. People who have a poor self-image, who view themselves negatively, or who are easily overwhelmed by life challenges may be more likely than others to experience depression. A serious loss, chronic illness, difficult relationship, or unwelcome change can also trigger it. There are three main types of depressive disorders: Major depression is a combination of symptoms that interfere with the ability to work, sleep, eat, and enjoy once pleasurable activities. These disabling episodes of depression can occur once, twice, or several times in a lifetime. Dysthymia causes long-term, chronic symptoms that do not disable individuals, but keep them from functioning at "full steam" or from feeling good. Sometimes, people with dysthymia also experience major depressive episodes. Bipolar disorder (manic-depression) is a chronic, recurring condition that includes cycles of depression and excessive elation, or mania. For a doctor to diagnose major depression in an individual, that person must exhibit five or more of the known symptoms during the same two-week period. Because depression has been shown to co-exist with other medical conditions, such as heart disease, cancer, or diabetes, and other psychiatric disorders, such as substance abuse, or anxiety disorders, seeking early diagnosis and treatment is crucial to recovery. Treatment may include antidepressant medications, psychotherapy, family therapy, electroconvulsive therapy (ECT), or a combination of those remedies. Without treatment, symptoms can persist for weeks, months, or years. Continued treatment may help prevent symptoms from recurring.