"He is 88 years old, he had a terrible case of bronchitis which then triggered a series of complications, partially brought on by the fact that he also has a form of Parkinson's disease," Becker said.
Becker also praised the medical care Bush is receiving as "unequaled anywhere," and asked those concerned to "keep him and the family in your prayers."
"I am thinking heaven has not seen such a barrage of prayer intentions since 'It's a Wonderful Life,' " Becker said.
Bush family spokesman Jim McGrath, meanwhile, said in a statement Friday that the former president "continues to improve."
"The President is alert and, as always, in good spirits -- and his exchanges with doctors and nurses now include singing," said McGrath. "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."
This post was updated at 2:04 p.m.