08/13/2012 03:25 pm ET Updated Aug 13, 2012

Bipolar II Disorder: Jesse Jackson, Jr. Is Being Treated For The Condition, But What Is It?

Congressman Jesse Jackson, Jr., the Democratic U.S. Representative for Illinois, is being treated for bipolar disorder, according to news reports.

The Chicago Tribune reported that Jackson has bipolar II depression, and is undergoing treatment at the Mayo Clinic.

The Mayo Clinic released a statement about Jackson's diagnosis:

Bipolar II disorder is a treatable condition that affects parts of the brain controlling emotion, thought and drive and is most likely caused by a complex set of genetic and environmental factors. Congressman Jackson underwent gastric bypass surgery in 2004. This type of surgery is increasingly common in the U.S. and can change how the body absorbs food, liquids, vitamins, nutrients and medications.

Jackson left for an unexplained medical leave in June. His wife, Sandi, earlier said that Jackson was experiencing "debilitating" depression, the Associated Press reported, quashing rumors that he had departed because of alcohol or drug problems.

In July, it was reported that Jackson was receiving treatment for a mood disorder, but there was no elaboration on exactly what kind of disorder it was.

Bipolar disorder, also known as manic-depressive disorder, includes symptoms of extreme mood swings where you may feel depressed one moment, and then extremely manic the next. These mood shifts can occur on a daily basis, or only several times a year, according to the Mayo Clinic.

Bipolar disorder is considered a mood disorder, along with depression, dysthymia (which is having a low, depressed mood for a year or more), substance-induced mood disorder (depression as a result of a substance like drugs or toxins) and mood disorder from a general medical condition (where a medical condition is the cause of the depressive symptoms), according to Ohio State University.

Mood disorders occur when brain chemicals are imbalanced, and can be contributed to by major negative life events, Ohio State University reported.

Bipolar disorder, in particular, is comprised of three different subtypes. The most severe kind is bipolar I disorder, which is when episodes of mania can reach dangerous levels, and the mood swings are evident enough to severely interfere with daily life, according to the Mayo Clinic. The least severe kind is cyclothymic disorder, which is when the depression and mania are not as severe.

Bipolar II disorder -- which is what Jackson has been diagnosed with -- on the other hand, usually involves less severe mania and longer periods of depression. Daily functioning may be affected, but not to the extent of bipolar I disorder, the Mayo Clinic reported.

The exact cause of bipolar disorder is not known, but researchers suspect genetics play a part, according to the National Institutes of Health.

Medications are usually used to treat bipolar disorder, though psychotherapy and some other kinds of treatments -- like electroconvulsive therapy -- may also be employed, according to the Mayo Clinic. Hospitalization is sometimes required.

Last year, actress Catherine Zeta Jones made headlines after she was treated for bipolar II disorder after helping her husband, Michael Douglas, through his throat cancer battle, People magazine reported.