(Bloomberg) — The U.K. charity Save the Children initiated an extraordinary review of safety procedures at an Ebola treatment center in Sierra Leone in an effort to understand how a Scottish nurse caught the lethal virus.

The nurse, 39-year-old Pauline Cafferkey, spent three weeks in December tending to Ebola patients at the treatment center Save the Children operates in Kerry Town, Sierra Leone. She is now in critical condition at the Royal Free Hospital in London. A Cuban doctor who became infected at the same center was flown to Geneva for treatment in November and has recovered.

The investigation is looking at how staff puts on and removes protective gear and at contact people have had inside the treatment center as well as outside, Save the Children’s Sierra Leone director Rob MacGillivray told the BBC in a video interview.

“Because of this very serious event, we have put in an extraordinary review to ensure that we do everything and leave no stone unturned to, as far as is possible, identify the source of this infection,” MacGillivray said. “We’re confident our protocols are working.”

Cafferkey, a U.K. National Health Service nurse, returned to Scotland late Dec. 28 and after feeling unwell and was placed in isolation at Gartnavel General Hospital, were she tested positive for Ebola. She was in stable condition before being transported to London, according to First Minister Nicola Sturgeon, the head of Scotland’s government.

Gradual deterioration

At the isolation unit in London, Cafferkey opted to receive blood plasma with antibodies from an Ebolasurvivor and an experimental anti-viral drug. Even so, her condition has gradually deteriorated and is now critical, the hospital said on Jan. 3.

The Kerry Town Ebola treatment center operates 80 beds and has cared for about 200 patients since it opened on Nov. 5. Save the Children runs the clinic as part of a U.K. government commitment to supply and support more than 1,000 beds in Sierra Leone, a former British colony.

More than 20,000 people have been infected in the world’s worst-ever Ebola outbreak, mostly in Sierra Leone, Liberia and Guinea, according to the World Health Organization. Among them were hundreds of health-care workers, including about 660 physicians.

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