In a brief email Wednesday, Jim McGrath, Bush's spokesman in Houston, said the 88-year-old former leader had been admitted Sunday to the ICU at Methodist Hospital. McGrath said Bush, the oldest living former U.S. president, was alert and talking to medical staff.
He said doctors are cautiously optimistic about Bush's treatment and that the former president "remains in guarded condition." He said Bush was surrounded by family.
Early Thursday, McGrath told The Associated Press he had no new information on Bush's condition and that he would release another statement "when events warrant it."
Bush has been hospitalized since Nov. 23, when he was admitted for a lingering cough related to bronchitis after having been in and out of the hospital for complications related to the illness.
A fever that kept Bush in the hospital over Christmas had gotten worse and doctors had put him on a liquids-only diet Wednesday following "a series of setbacks.'
"It's an elevated fever, so it's actually gone up in the last day or two," McGrath told The Associated Press. "It's a stubborn fever that won't go away."
But he said the cough that initially brought Bush to the hospital had improved.
Bush was visited on Christmas by his wife, Barbara, his son, Neil, and Neil's wife, Maria, and a grandson, McGrath said. Bush's daughter, Dorothy, was expected to arrive Wednesday in Houston from Bethesda, Md. The 41st president has also been visited twice by his sons, George W. Bush, the 43rd president, and Jeb Bush, the former governor of Florida.
Bush and his wife live in Houston during the winter and spend their summers at a home in Kennebunkport, Maine.
The former president was a naval aviator in World War II - at one point the youngest in the Navy - and was shot down over the Pacific. He achieved notoriety in retirement for skydiving on at least three of his birthdays since leaving the White House in 1992.