From the Royal College of Psychiatrists :
http://www.rcpsych.ac.uk/mentalhealthinfo/problems/bipolardisorder/bipolardisorder.aspx
How common is bipolar disorder?
About 1 in every 100 adults has bipolar disorder at some point in their life. It usually starts during or after the teenage years. It is unusual for it to start after the age of 40. Men and women are affected equally.
What types are there?
Bipolar I
There has been at least one high, or manic episode, which has lasted for longer than one week.
Some people with Bipolar I will have only manic episodes, although most will also have periods of depression.
Untreated, manic episodes generally last 3 to 6 months. Depressive episodes last rather longer - 6 to 12 months without treatment.
Bipolar II
There has been more than one episode of severe depression, but only mild manic episodes – these are called ‘hypomania’.
Rapid cycling
More than four mood swings happen in a 12 month period. This affects around 1 in 10 people with bipolar disorder, and can happen with both types I and II.
Cyclothymia
The mood swings are not as severe as those in full bipolar disorder, but can be longer. This can develop into full bipolar disorder.
What causes bipolar disorder?
We don't understand this well, but research suggests that:
Bipolar disorder runs in families - it seems to have more to do with genes than with upbringing.
There may be a physical problem with the brain systems which control our moods - this is why bipolar disorder can often be controlled with medication.
Episodes can sometimes be brought on by stressful experiences or physical illness.