Former US President Bill Clinton will remain in hospital overnight, his spokesman said on Twitter, adding that Mr Clinton's health indicators were "trending in the right direction".
The 75-year-old, who left office in 2001, entered the University of California Irvine Medical Center on Tuesday evening for the infection, according to his spokesman, Angel Urena.
Mr Urena also said that his white blood cell count was down significantly, indicating a drop in the infection.
"In order to receive IV antibiotics, he will remain in the hospital overnight," Mr Urena said.
Mr Clinton was "up and about, joking and charming the hospital staff", Mr Urena told Reuters.
Mr Clinton went to the hospital after feeling fatigued and was diagnosed with an infection of the bloodstream that doctors believe started as a urinary tract infection, CNN reported, citing his doctors.
The former president's physicians, Alpesh Amin and Lisa Bardack, said he was "admitted to the hospital for close monitoring and administered IV antibiotics and fluids".
"He remains at the hospital for continuous monitoring," they said in a statement. "After two days of treatment, his white blood cell count is trending down and he is responding to antibiotics well."
They added: "We hope to have him go home soon."
CNN cited doctors saying Mr Clinton was in the intensive care unit primarily to give him privacy.
Mr Clinton, a Democrat who was president from 1993-2001, has had past health issues, including a 2004 quadruple bypass surgery and a 2010 procedure to open a blocked artery in his heart with two stents. CNN reported that Mr Clinton's current hospital stay was not related to his heart issues.
The former Arkansas governor came to the White House by defeating an incumbent president, Republican George H.W. Bush, and served during a period of acute partisanship in Washington, a harbinger of the current bitter political state.
He won re-election in 1996 against Republican Senator Bob Dole.
Mr Clinton endured bruising political battles with Republicans. He was impeached in 1998 by the Republican-led House of Representatives over his sexual relationship with White House intern Monica Lewinsky, but remained in office when the Senate acquitted him in 1999.
He is known for a remarkable talent for connecting with people and an exceptional understanding of policy issues.
Mr Clinton morphed into a political husband after leaving office, when his wife Hillary Clinton was elected US senator from New York in 2000. She unsuccessfully sought the 2008 Democratic presidential nomination and won the party's 2016 nomination but lost the election to Donald Trump even though she received close to three million more votes.
