A volunteer British nurse who contracted Ebola was in a “critical” condition Sunday, as the new UN mission chief on the disease insisted that ending the deadliest-ever outbreak was within reach.

Nurse Pauline Cafferkey’s health has taken a turn for the worse in recent days, the Royal Free Hospital in London said in a statement.

“The condition of Pauline Cafferkey has gradually deteriorated over the past two days and is now critical,” the Royal Free Hospital said in a statement.

On Wednesday, doctors had said the 39-year-old Scot, who had been working with the charity Save the Children in Sierra Leone, was sitting up in bed, reading and talking to staff from inside her isolation tent in the hospital.

They said Cafferkey had agreed to be treated with blood plasma from an Ebola survivor, containing virus-fighting antibodies, and also take an experimental anti-viral drug.

Cafferkey was volunteering at a British-built treatment centre in Kerry Town, not far from Sierra Leone’s capital Freetown, when she contracted the deadly virus.

British Prime Minister David Cameron told BBC television that Ebola was “certainly the thing uppermost in my mind today with Pauline Cafferkey in hospital, and all of us are thinking of her and her family.

“And also how incredibly brave these people are, not only doctors and nurses from our NHS (National Health Service) but also people from our armed forces who have been working in west Africa in very difficult conditions,” he added.

Expert microbiologist Professor Hugh Pennington said Cafferkey would need luck to survive the deadly virus, which experts still do not fully understand.

“The plasma is probably her best chance of treatment,” he said of the nurse, who was diagnosed in Glasgow on December 29 after flying home.

Medical professionals have been particularly affected by the worst Ebola outbreak on record. Out of 678 healthcare workers known to have contracted the virus, 382 have died.