HOUSTON -- Former President George H.W. Bush, suffering a stubborn, worsening fever, has been moved to the intensive care unit at a Houston hospital where he has been a patient for more than a month.

Bush's spokesman, Jim McGrath, said Bush was admitted to the ICU on Sunday at Methodist Hospital, "following a series of setbacks, including a persistent fever."

McGrath says Bush, 88, was alert and conversing with medical staff, and that doctors are cautiously optimistic about his treatment. No other details about his medical condition were provided.

Earlier Wednesday, McGrath said a fever that kept Bush in the hospital over Christmas had gotten worse and that doctors had put him on a liquids-only diet.

A bronchitis-like cough initially brought Bush to the hospital in late November. McGrath says the cough has improved.

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Bush was visited on Christmas by his wife, Barbara, son Neil and Neil's wife, Maria, and a grandson, McGrath said. Bush's daughter, Dorothy, was on her way Wednesday from Bethesda, Md. The 41st president has also been visited twice by sons George W. Bush, the 43rd president, and Jeb Bush, 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 sky diving on at least three of his birthdays since leaving the White House in 1993.