Doctors warn that the main threat facing Pope Francis is sepsis, an infection of the blood that can occur as a complication of pneumonia.

The Vatican described his condition as critical for the first time on Saturday, reporting that the 88-year-old Francis needed supplemental oxygen and blood transfusions after a "prolonged asthma-like respiratory crisis.

“Pope Francis is touched by the numerous messages of affection and closeness that he continues to receive in these hours,” the Vatican statement said. The pope expressed particular gratitude for the well wishes he received from his fellow patients in Rome’s Gemelli Hospital and asked for continued prayers.