Former President George H. W. Bush remained in the intensive care unit of a Houston hospital on Friday, but "continues to improve" and his exchanges with medical staff now include singing, according to a statement from a Bush family spokesman.

Bush, 88, was admitted to Methodist Hospital on Nov. 23 for bronchitis. He was transferred to intensive care on Sunday after setbacks including a persistent fever.

"President Bush remains in the intensive care unit at Methodist Hospital, where he continues to improve," family spokesman Jim McGrath said in a statement. "The president is alert and, as always, in good spirits -- and his exchanges with doctors and nurses now include singing."

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The statement said that the Bushes, like their doctors, are "cautiously optimistic that the current course of treatment will be effective." The Bushes thank everyone for their prayers and good wishes, McGrath said.

Bush has lower-body parkinsonism, which causes a loss of balance, and has used a wheelchair for more than a year. 

Bush was the 41st U.S. president and is the father of former President George W. Bush. In a political career spanning four decades, Bush, a Republican, also served as a congressman, ambassador to the United Nations, envoy to China, CIA director, and vice president for two terms under Ronald Reagan.