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CHICAGO - Rep. Jesse Jackson Jr. is undergoing treatment for bipolar disorder, doctors at the Mayo Clinic in Minnesota announced Monday.

According to a statement from the clinic, Jackson "is responding well to the treatment and regaining his strength."

The announcement by the clinic comes two months after Jackson, the son of civil rights leader Jesse Jackson, abruptly left Congress. Initially Jackson's office said the congressman, who represents a district that includes a large portion of Chicago's South Side and southeast suburbs, was taking a medical leave of absence to seek treatment for "exhaustion." But two weeks later his office said his condition was "more serious than we thought."

For weeks, Jackson's specific condition was unknown, sparking concerns from both his colleagues and his constituents. Illinois Sen. Dick Durbin warned that Jackson needed to update his condition "soon." As the summer went on, Jackson's office gradually released more information, culminating in today's announcement by the Mayo Clinic.

Jackson had spent time this summer at a facility in Arizona before being transferred to the Mayo Clinic in Rochester, Minn. In its statement, the clinic noted that the Illinois congressman underwent gastric bypass surgery eight years ago and such an operation "can change how the body absorbs food, liquids, vitamins, nutrients and medications." Bipolar II disorder, the clinic said, "affects parts of the brain controlling emotion, thought, and drive."

However, there is no data to suggest a causative link between gastric bypass surgery and bipolar disorder in terms of worsening the problem. The disease, which affects approximately 5.7 million Americans, is characterized by cycles of depression and elation or mania, with mood changes both slow and rapid. The types of mania involved with the disorder can lead to significant impairment, even increasing the risk for suicide. Bipolar II disorder, the type Jackson suffers from, is less severe than the type 1 form of the disorder; it involves milder forms of mood elevation and milder episodes of hypomania that alternate with periods of severe depression.

Jackson has endured a difficult time over the past several years. He is the target of a probe by the House Ethics Committee into possible wrongdoing in how President Barack Obama's Senate seat - now held by Sen. Mark Kirk - was filled. Only days before Jackson left Congress on medical leave, one of his former fundraisers, Raghuveer Nayak, was arrested by the FBI on charges of paying hundreds of thousands of dollars in bribes to doctors.

In addition, Jackson has faced turmoil at home. Last year his wife Sandi revealed that Jackson had engaged in an extramarital affair.

Last Thursday his wife told reporters that there was no timetable for Jackson's return to Congress.

"At this point we are taking every day one day at a time," she said. "But we here on the ground are preparing for his eventual return. We don't know when that's going to be, but we want his constituents to know that they are very much on his mind.

Jackson is involved in a re-election race this fall. On the same day of his wife's comments, Jackson's spokesman Rick Bryant said the congressman could resume work within weeks, noting that he seemed "like his old self."

ABC News' Dr. Tiffany Chao contributed to this report.

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