Pope Francis is responding “positively” to medical treatment for pneumonia, according to the Vatican, a week after the 88-year-old pontiff was hospitalized in Rome.

Pope Francis is being treated in hospital for double pneumonia — but what is this condition, and why might the pope be particularly vulnerable to it?

His doctors said “the pope is fine” and “working,” though they cautioned he remains in significant danger as he fights a bout of double pneumonia.