Former president George H.W. Bush was being treated for a blood infection on Tuesday, just days after laying his wife Barbara to rest.
His office stressed late Monday that Bush, 93, was responding to treatment.
He was admitted to Houston Methodist Hospital early Sunday, the day after bidding a final farewell to his wife of 73 years during a funeral in the same Texas city.
Barbara Bush died last Tuesday.
The 41st president greeted and shook hands with numerous attendees at the invitation-only funeral attended by four former presidents, including himself and his son George W. Bush.
More than a thousand people attended the ceremony.
"I always knew Barbara was the most beloved woman in the world, and in fact I used to tease her that I had a complex about that fact," Bush said after his wife's passing.
Their marriage was the longest presidential union in American history.
He married the love of his life while on leave from military deployment in 1945, several months before World War II ended.
Bush, who suffers from Parkinson's disease and is wheelchair-bound as a result, has been repeatedly hospitalized for pneumonia. His is a less-common version of Parkinson's that only affects the lower body.
"President Bush was admitted to Houston Methodist Hospital... after contracting an infection that spread to his blood," a statement from his office read.
"He is responding to treatments and appears to be recovering. We will issue additional updates as events warrant."
George Herbert Walker Bush was born June 12, 1924 in Milton, Massachusetts to a wealthy New England political dynasty, the son of Prescott Bush, a successful banker and US Senator for Connecticut.
Bush had a pampered upbringing and attended the prestigious Phillips Academy in Andover, but delayed his acceptance to Yale in order to enlist in the Navy on his 18th birthday and head off to World War II.
He went on to fly 58 combat missions during the war. He was shot down over the Pacific by Japanese anti-aircraft fire and rescued by a submarine after huddling in a life raft for four hours while enemy aircraft circled.
He served as vice president to Ronald Reagan for eight years after losing to him in the 1980 Republican primary, and was elected president by a solid margin in 1988, as the Cold War was coming to an end.
The decorated war hero and former CIA chief suffered the ignominy of being a one-term president, but led a political dynasty that would win back the White House eight years later, albeit with a leader with a very different approach.
A foreign policy realist who navigated the turbulent fall of the Soviet Union in 1989, he assembled an unprecedented coalition to defeat Saddam Hussein two years later.
But he also presided over a period of economic malaise and infuriated his fellow Republicans during a budget battle with rival Democrats by famously breaking his vow: "Read my lips: no new taxes."
After retiring from public life, Bush fulfilled a wartime pledge to one day jump out of a plane for fun and went skydiving on his 75th, 80th and 85th birthdays.
He joined Bill Clinton -- the Democrat to whom he lost his re-election bid -- to raise funds for victims of the 2004 Asian tsunami and the 2010 Haiti earthquake. In February 2011 Democratic president Barack Obama awarded Bush the highest US civilian honor, the Medal of Freedom.
Bush, who served as commander-in-chief from 1989 to 1993, is the oldest of the five living former US presidents.
He is father to ex-president George W. Bush (2001-2009) and former Florida governor Jeb Bush, who was a contender in the 2016 Republican presidential primaries.