JACKSONVILLE, Fla. -

Former president George H. W. Bush has been admitted to the intensive care unit at a Houston, Texas hospital.

The 41st president has been in the hospital since November 23rd for a lingering cough related to bronchitis.

A Bush spokesperson says the former president, 88, has suffered a series of set backs and was recently moved into ICU from normal care.

Doctors say there could be many reasons why the former president was moved to ICU. A local doctor at Shands Jacksonville said a doctor's first priority is finding the underlying cause of the fever.

"He's surrounded by his family," Spokesman Jim McGrath said. "That's the best medicine and it's not the ideal way to spend Christmas, but he's playing the hand he's been dealt."

Bush was checked in to be treated for a lingering cough caused by bronchitis and was expected to be released before Christmas. Doctors changed their minds and moved him to intensive care on Sunday.

"He's had a series of set backs that have kind of changed the time line of his release," McGrath said. "But his doctors are cautiously optimistic that they have his current issue under control."

With an elevated fever, the former president is on a liquids only diet.

Doctors continue to treat his fever with tylenol but still haven't nailed down a cause. It's what Dr. DeVos of Shands Jacksonville said is the first thing doctors do when a patient develops a fever.

"We may do x-rays or laboratory test," Emergency Medicine Physician Dr. Elizabeth DeVos said. "Looking for the source of the infection or the infection in the blood stream."

Dr. Devos said it's hard to nail down exactly why the former president was moved to ICU.

"It really depends on the situation," Dr. DeVos said. "The intensive care unit is able to frequent rechecks, so some patients need to be in intensive care unit for frequent monitoring. Some patients need very intensive nursing or wound care or the underlying condition like pneumonia."

The family spokesman said the former president remains upbeat as he continues to take visitors.

Wednesday Bush's daughter Dorothy arrived for a visit. His sons, former President George W. Bush and former Florida Governor Jeb Bush will pay a visit in the coming days.