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?

LONDON — Vatican officials said late Tuesday that Pope Francis had developed pneumonia in both lungs, further complicating the pontiff’s recovery. The 88-year-old pope was hospitalized Friday after a weeklong bout of bronchitis worsened. On Monday, doctors said he had developed a mix of infections in his respiratory tract.

The Vatican said Pope Francis has the "onset of bilateral pneumonia," and that tests, a chest X-ray and the pope's clinical condition present a "complex picture."