Double pneumonia is a serious infection that can inflame and scar both lungs and makes breathing more difficult.

Candles with the pictures of Pope Francis are the laid under the statue of late Pope John Paul II outside Agostino Gemelli Polyclinic in Rome. Source: AAP / Gregorio Borgia/AP

The Vatican had said previously that the pope had a polymicrobial infection, which occurs when two or more micro-organisms are involved, adding that he would stay in hospital as long as necessary to tackle a “complex clinical situation”.

‘Hasn’t lost his sense of humour’

“We will pray for him so that he can recover as soon as possible,” said Gianfranco Rizzo, a pilgrim from Bari, Italy.

The Vatican’s top diplomat, Cardinal Pietro Parolin, returned to Rome as scheduled on Wednesday from a trip to Burkina Faso.