HOUSTON - Former President George H.W. Bush remained in intensive care at a Houston hospital on Friday but his condition continues to improve, a spokesman said.

"The President is alert and, as always, in good spirits - and his exchanges with doctors and nurses now include singing," family spokesman Jim McGrath said in a brief statement.

Bush, 88, the nation's oldest living former president, was admitted at Methodist Hospital in Houston on Nov. 23 because of a bronchitis-related cough, after spending about a week there that month for the same condition.

The cough was mostly resolved by the time he was moved to intensive care Sunday for treatment of a fever that doctors were having difficulty controlling.

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

On Thursday, a longtime Bush aide tried to quell concern about Bush's condition by saying the former president likely would advise well-wishers to "put the harps back in the closet."

Jean Becker, Bush's Houston chief of staff, said the former president would likely be in the hospital for a while, noting his age and that "he had a terrible case of bronchitis which then triggered a series of complications."

Updates on his health are being limited out of respect for the family, she said.