Bush family spokesman Jim McGrath said Bush, 88, was moved to the intensive care unit Sunday and has been kept on a liquids-only diet, the Houston Chronicle reported Tuesday.
Doctors are treating the fever with acetaminophen and remained unable to determine the cause, but McGrath said the former president is upbeat, the newspaper reported.
"All the kids are making their own plans to come back to Houston to visit him and help him rally -- rally to recovery," McGrath said.
The 41st president was initially hospitalized Nov. 23 for a persistent cough due to bronchitis. He was kept in the hospital after taking a turn for the worse, but doctors said in early December his condition was not life-threatening.