Several studies done in different American communities have found that about 5% of the population can be diagnosed as having major depression at any one point in their lives. At least 10% of the population will experience a major depression
during their lifetime, with some studies finding rates as high as
25%. Women are at least one and a half times more likely to
become depressed than men. Studies in many countries and cultures
and across all social classes show similar frequencies of
depression. People with a history of serious depression have, on
the average, about five episodes during their lifetimes, although the
number of episodes varies greatly; some people will have only one
whereas others will have many more. Early treatment can decrease the length and severity of depression for most people.
Manic Depression
At least 10% of people with depression will also have manic episodes. They are said to have “manic-depressive” or “bipolar” disorder.
During mania, mood changes from its normal level to an elevated,
expansive, elated, or even euphoric state that patients often describe
as being “on top of the world”. During a manic episode
a person may sleep very little, talk continually and very rapidly, take
little time to eat, and experience racing thoughts. At times
mania is characterized by irritability, impatience, and discomfort
rather than elation and euphoria and often manic and depressive
symptoms are mixed together (mixed or dysphoric mania). Sometimes
the manic state progresses to a point where judgement is impaired and
contact with reality is lost. It may be difficult to understand
what a person is saying. Sometimes poorly thought-out decisions
are acted on impulsively, with serious financial, social, or
occupational consequences for self, family, and others.
Hospitalization may be necessary.

What Are The Chances I Will Develop Depression?
To summarize, depression is widespread and common.
Serious depression affects 5% of the population at any one time, and at
least 10% of the population at some point in their lifetime. At
least 10% of people with major depression end their lives by suicide.
Although depression spares no segment of the world’s population, treatment can restore many sufferers to lives of sensitivity, creativity, and accomplishment.
Many factors are thought to contribute to the development of depression.
Depression probably appears when several factors combine in particular
ways that are not yet well understood. The occurrence of many
different combinations partly explains the many different forms of
depression.