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?

Francis was admitted to Rome's Gemelli hospital on Feb. 14 after a case of bronchitis worsened; doctors later diagnosed a complex respiratory infection, involving bacteria, virus and other organisms and the onset of pneumonia in both lungs on top of asthmatic bronchitis. They prescribed “absolute rest.”