Former US president George HW Bush has been admitted to a hospital intensive care unit "following a series of setbacks including a persistent fever", but he is alert and talking to medical staff, his spokesman said.
Jim McGrath, Mr Bush's spokesman in Houston, said in a brief email that Mr Bush was admitted to the ICU at Methodist Hospital on Sunday.
He said doctors are cautiously optimistic about his treatment and that the former president "remains in guarded condition".
No other details were released about his medical condition, but Mr McGrath said Mr Bush is surrounded by family. Mr Bush has been in hospital since mid-November.
Earlier, Mr McGrath said a fever that kept Mr Bush in the hospital over Christmas had got worse and that doctors had put him on a liquids-only diet.
"It's an elevated fever, so it's actually gone up in the last day or two," Mr McGrath said. "It's a stubborn fever that won't go away."
But he said the bronchitis-like cough that initially brought the 88-year-old to the hospital has improved.
Mr Bush was visited on Christmas by his wife, Barbara, his son, Neil, and Neil's wife, Maria, and a grandson, Mr McGrath said. Mr Bush's daughter, Dorothy, was expected to arrive in Houston from Bethesda, Maryland.
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.
Mr 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 the Second World War - at one point the youngest in the Navy - and was shot down over the Pacific. He achieved headlines in retirement for skydiving on at least three of his birthdays since leaving the White House in 1992.