Here's a look at what we know about the operation and what the pope's doctors will be watching out for.

The mission of the Vatican hospital is "today more aimed at profit than on caring for children," according to a report from the AP.

The new procedures are intended to fix problems linked to mitochondria, the energy-producing structures outside a cell's nucleus.

The U.N. agency took the rare step despite a lack of definitive evidence proving the mosquito-borne virus is causing a surge in babies born with brain defects and abnormally small heads in Brazil and following a 2013-14 outbreak in French Polynesia.