His spokesman Jim McGrath said the former president's fever rose on Wednesday, but doctors at Methodist Hospital said he is doing better than the day before and was alert and talking to medical staff.
He was admitted to the ICU on Sunday and since has been on an all-liquid diet.
"He is considered in guarded condition," McGrath said. "He has suffered a series of setbacks."
The fever is being treated with Tylenol, but it's not responding to the treatment. McGrath was not sure what caused the fever.
McGrath said Bush is alert, having a "running banter" with his nurses and is surrounded by family. "His doctors have said emphatically this is not the end game."
The 41st president was admitted to Methodist Hospital in late November for a bronchitis-related cough and has remained there to regain strength.