Pope Francis in critical condition, Vatican says

Vatican News reported that medical professionals say the pope is not "in danger of death," but is not completely "out of danger" and will need to be hospitalized for "at least" all of next week. Physicians are worried about the threat of sepsis, a dangerous infection that can be fatal. 

Fox News senior medical analyst Dr. Marc Siegel told "Fox News Live" that he is "looking very closely at the next 12 hours," as he says that time is critical.

"This is a very common complication in this kind of complication, especially since he has underlying bronchitis and problems with the upper airways as well," Siegel said. 

He also noted that, given Francis' medical history, it is "very, very likely" that he would get pneumonia.However, despite the complications the pontiff is facing, Dr. Siegel says he is "not in any way pessimistic" about the situation.

The 88-year-old pontiff was admitted to Rome's Gemelli Hospital on Feb. 14 after experiencing respiratory issues that were thought to be related to bronchitis at the time. Doctors later diagnosed him with double pneumonia.

"The Church is governed using the head and the heart, not the legs," the pope wrote.