Pope Francis has the onset of double pneumonia, the Vatican said on Tuesday, complicating treatment for the 88-year pontiff and indicating a further deterioration in his fragile health.

As Pope Francis battles dangerous lung infection, here’s what you need to know about the causes, symptoms, prevention tips and treatment of double pneumonia.

The Vatican says Pope Francis will remain hospitalized after a chest scan of the 88-year-old pontiff shows he is battling double pneumonia, an infection in both lungs. It comes as people from all over the world have gathered outside the hospital to offer their prayers to him.