The aide, who spoke to reporters at the hospital on the condition his name wasn't used, did not elaborate on the reason Clinton was in the ICU. He said Clinton had a urological infection that spread to his bloodstream, but is on the mend and never went into septic shock, a potentially life-threatening condition.
Clinton, 75, was admitted to the hospital on Tuesday evening for a non-COVID-related infection, his spokesman Angel Ureña said in a Thursday statement.
On Friday afternoon, Ureña said all health indicators "are trending in the right direction," including the former president's white blood count which has decreased significantly.
Clinton had been expected to be released from the hospital as early as Friday, but Ureña said he would remain in the hospital overnight again to receive further IV antibiotics.
"President Clinton continues to be in excellent spirits, and is deeply grateful for the outstanding care he is receiving and the well wishes that people have sent from across America and the around the world," his spokesman said in a tweet.
The former president was visited late Thursday by his wife, former first lady and Secretary of State Hillary Clinton, who was later seen leaving the medical center with her Secret Service detail.
President Joe Biden said Friday during remarks at the University of Connecticut that he had spoken to Clinton and the former president "sends his best."
"We're all thinking about President Clinton today," he said. "He's always been the comeback kid."
Later, Biden told reporters that Clinton is "not in any serious condition" and is "getting out shortly."
A second statement from Clinton's spokesman quoted physicians Dr. Alpesh Amin and Dr. Lisa Bardack, who said the former president has been "administered IV antibiotics and fluids."
"After two days of treatment, his white blood cell count is trending down and he is responding to antibiotics well," the doctors said. "The California-based medical team has been in constant communication with the President's New York-based medical team, including his cardiologist. We hope to have him go home soon."
In the years since Clinton left the White House in 2001, the former president has faced health scares. In 2004, he underwent quadruple bypass surgery after experiencing prolonged chest pains and shortness of breath. He returned to the hospital for surgery for a partially collapsed lung in 2005, and in 2010 had a pair of stents implanted in a coronary artery.
He responded by embracing a largely vegan diet that saw him lose weight and report improved health.
He repeatedly returned to the stump, campaigning for Democratic candidates, mostly notably his wife, Hillary, during her failed 2008 bid for the presidential nomination. And in 2016, as Hillary Clinton sought the White House as the Democratic nominee, Bill Clinton - by then a grandfather and nearing 70 - returned to the campaign trail.
A spokesman for the Clinton Foundation said the former president was in the Los Angeles area for private events related to his charitable organization.
The Associated Press contributed to this report.