Concerns over Pope Francis' health have mounted in recent years as have questions about his succession. Here's what we know.

There is no age restriction for a Pope, but history shows they are usually chosen at an advanced age. Pope Francis was 76 when elected and is now 88. Before him, Benedict XVI took the position at 78 and stepped down at 85, becoming the first Pope in nearly 600 years to resign.

Pope Francis, battling double pneumonia, remains in a critical condition and his prognosis is still guarded because of the "complexity" of his clinical condition, the Vatican said on Sunday. The pope was admitted to Rome's Gemelli hospital on February 14 after having trouble breathing for several days and subsequently had pneumonia diagnosed in both lungs.