But his staff gave no hint of the congressman's whereabouts or exactly what he was suffering from, saying only that he has grappled with physical and emotional problems "privately for a long period of time" and was at an inpatient facility.
The nine-term Chicago Democrat has come under intense scrutiny in recent years for ties to imprisoned former Illinois Gov. Rod Blagojevich and an extramarital affair. Despite a pending U.S. House ethics investigation connected to Blagojevich, he has been widely expected to win re-election in November.
He has been on medical leave for three weeks, though his office didn't disclose his leave until last week - and did so in a meager three-sentence statement saying he was being treated for exhaustion. Neither his family nor his staff has offered more explanation, and the statement Thursday provided few details.
"Congressman Jackson's medical condition is more serious than we thought and initially believed," the statement said. "Recently, we have been made aware that he has grappled with certain physical and emotional ailments privately for a long period of time. At present, he is undergoing further evaluation and treatment at an in-patient medical facility."
The statement said Jackson, 47, will need "extended in-patient treatment as well as continuing medical treatment thereafter."
Multiple messages left for his spokesmen, his brother and his wife, Chicago Alderman Sandi Jackson, were not returned. His father, Rev. Jesse Jackson, said he spoke to his son once recently and he sounded "exhausted and overwhelmed."
"He'll be in for a longer stay for more evaluations and treatment of his challenges," the civil rights leader said, declining to elaborate.
The news of Jackson's illness has baffled congressional colleagues. They noted his absence on the House floor in previous weeks, but several said they have not heard from him at all.
"No one has a clue," said U.S. Rep. Mike Quigley, a fellow Illinois Democrat. "There is concern among his colleagues ... Anybody there could understand that. It's a stressful occupation."
In order to take a medical leave of absence, Jackson must notify the leader of his party's House delegation in writing. A spokesman for Democratic Leader Nancy Pelosi, D-Calif., did not immediately respond to multiple requests for comment on Thursday. Michael Steel, a spokesman for House Speaker John Boehner, R-Ohio, said his office had not received any notification from Jackson's office.
Jackson, who first won office in 1995, is facing a House Ethics Committee investigation over allegations that he was involved in discussions about raising money for Blagojevich's campaign in exchange for the then-governor appointing him to President Barack Obama's vacated U.S. Senate seat.
Jackson also allegedly directed fundraiser and longtime friend Raghuveer Nayak to buy plane tickets for a woman described as Jackson's "social acquaintance." Jackson has since called the incident a personal matter that he and his wife have dealt with.
Nayak was arrested last month and pleaded not guilty to unrelated fraud charges involving outpatient surgery centers he owns. At Blagojevich's first corruption trial in 2010, prosecutors said another Blagojevich fundraiser was ready to testify that Jackson instructed Nayak to raise money for Blagojevich's campaign to help him secure the Senate seat. The same witness later testified that he attended a meeting with Jackson and Nayak.
Jackson never has been charged and has denied any wrongdoing. But he's had to repeatedly defend himself, especially on the campaign trail in recent years.
He had to campaign harder in the March primary than he has in past years and faced a serious primary from former U.S. Rep. Debbie Halvorson, who made Jackson's ethical troubles a focus of her campaign. Jackson faces two opponents in November and both said Jackson owes the public more information.
"There is an obligation here, especially considering there are a lot of rumors going around," college professor Brian Woodworth, a Republican, said Thursday. "There's an obligation they have to let people know what's going on. There's still an air of mystery."
Postal worker Marcus Lewis, who is challenging Jackson as an independent, agreed. He said the lack of details about Jackson's illness only contributed to the rumors, and he questioned the timing of Jackson's announcement last week, which came just before the deadline for independent and new party candidates to file candidacy papers.
"Because you are a public servant, tell us the facility or the treatment center where you are. Don't go under the ground," he said. "You need to let your constituents know where you are. This is getting ridiculous."
Jackson spokesman Rick Bryant has said that Nayak's arrest had nothing to do with the medical leave and that Jackson's congressional offices remain open.
The congressman has kept medical information under wraps before. He underwent weight loss surgery in 2005, but he disclosed it months later after questions were asked about a nearly 50-pound weight loss.
Jackson represents Illinois' 2nd District, which traditionally includes neighborhoods on Chicago's South Side and in its south suburbs. The district was expanded this year to include more territory south of the metropolitan area that isn't as familiar with him.
Reaction in Jackson's district, where voters have overwhelmingly supported him each election, was a mix between people wanting to know more and wanting to respect the congressman's privacy.
"I want to be fair, but we find out he's ill so soon after his friend was charged," said Frances West, 74, who was at a local post office. "I said, 'That sure does sound strange.' At the same time he's been under a lot of stress (for years) as a congressman."
Ann Baker, 60, who was eating lunch at a restaurant frequented by Jackson - a calendar with his picture hangs on the wall - said it was a personal matter.
"He's done a great job for us for so long," she said. "But people speculate. We all do it. Nine out of 10 times there's nothing behind it. We're getting enough information. He's human and we all have problems."