Former president George Bush Sr is in "good spirits" amid signs of recovering health, even singing with doctors and nurses treating him at a Houston hospital, his spokesman said on Friday.
The 88-year-old was admitted to the Methodist hospital's intensive care unit on Sunday, after a series of setbacks including a persistent fever.
But in the latest update on his condition, the president's spokesman Jim McGrath said Bush "continues to improve" although he remains in intensive care.
"The president is alert and, as always, in good spirits – and his exchanges with doctors and nurses now include singing," McGrath said.
He added that doctors and the Bush family remained "cautiously optimistic that the current course of treatment will be effective".
Bush, the oldest living former president, was admitted to hospital on 23 November for a stubborn cough related to bronchitis, after being in and out of hospital after complications linked to the illness.
On Wednesday, it was disclosed that a fever that had kept Bush in the hospital over Christmas had worsened, 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," McGrath said. "It's a stubborn fever that won't go away."
But he said the cough that initially brought Bush to the hospital has improved.
A hospital spokesman said in mid-December that Bush was expected to be home in time for Christmas, but, according to Reuters, the spokesman later said doctors felt he should build up his strength before returning home.
Bush has lower-body Parkinsons, which causes a loss of balance, and has used wheelchairs for more than a year, McGrath told Reuters in an email.
After serving as vice-president under Ronald Reagan, Bush took office as the 41st president in 1989 and served a single term in the White House. He was a naval aviator in the second world war – at one point the youngest in the US navy – and was shot down over the Pacific. After leaving the White House in 1992, he celebrated at least three of his birthdays by skydiving, and had threatened to celebrate his 90th the same way.
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