Pauline Cafferkey, who had been helping in the battle against the virus in the West African country, is being treated in a specialist isolation unit at the Royal Free Hospital in London.
A brief statement from the hospital issued yesterday said: “The Royal Free London NHS Foundation Trust is sorry to announce that the condition of Pauline Cafferkey has gradually deteriorated over the past two days and is now critical.”
Just two days previously, medics had reported Cafferkey was sitting up in bed, talking and communicating with her family.
But Dr Michael Jacobs, infectious diseases consultant at the Royal Free, had warned that she faced a “critical” few days while being treated with an experimental anti-viral drug which is “not proven to work”, along with blood plasma from a survivor.
The hospital, where she has been treated in isolation since Tuesday, was unable to obtain ZMapp, the drug used to treat recovered British nurse William Pooley, because “there is none in the world at the moment”.
Pooley, from Suffolk, was flown to London to be treated in the Royal Free after being diagnosed with ebola in Sierra Leone, where he was working as a volunteer. He was treated with ZMapp and returned to West Africa to resume his work after recovering from the virus.
A Norwegian woman infected with ebola in Sierra Leone was given the last remaining dose of ZMapp in the world in October. The manufacturers have said it will take months to make more of the experimental drug.
Yesterday, First Minister Nicola Sturgeon said: “Our thoughts continue to be with Pauline Cafferkey and her family during this extremely distressing time.
“I would like to thank all of the health professionals involved in treating Pauline, as they continue to show tremendous dedication and expertise.”
Prime Minister David Cameron said on Twitter that his “thoughts and prayers” are with Pauline.
Pauline Cafferkey, from Glasgow, is the first person to be diagnosed with ebola in the UK. She was part of a 30-strong team of medical volunteers deployed to Africa by the UK Government last month and had been working with Save the Children at the Ebola Treatment Centre in Kerry Town, Sierra Leone.
Cafferkey was initially placed in isolation at a Glasgow hospital early on Monday after feeling feverish, before being transferred south on an RAF C-130 Hercules plane.
The healthcare worker had flown from Sierra Leone via Morocco to Heathrow, where she was considered high-risk because of the nature of her work, but showed no symptoms during screening.
While waiting at Heathrow for a connecting flight to Glasgow Cafferkey raised fears about her temperature and was tested a further six times in the space of 30 minutes.
Despite her concerns, she was given the all-clear and flew on to Scotland where, after taking a taxi home, she later developed a fever and raised the alarm.
The UK Government’s chief medical officer, Dame Sally Davies, admitted questions have been raised about the airport screening procedure for ebola.
On Friday, it was confirmed all UK-based passengers and crew aboard the two flights taken by Cafferkey had been contacted by medical authorities and given advice.
Ebola has claimed around 7000 lives during the current epidemic in West Africa. It is transmitted by direct contact with the bodily fluids of an infected person.
Leading infections expert Hugh Pennington, emeritus professor of bacteriology at Aberdeen University, said detecting people who are on the verge of falling ill with the virus is difficult.
But he added: “The good news is that people in that state are not infectious and even if she had been ill on the plane, the likelihood of transmission would be very low.
“It is not like flu where you can breathe in the virus, it is not transmitted that way – it has to be contact with body fluids basically.”
Last week, a woman who had recently returned from Sierra Leone was taken to Aberdeen Royal Infirmary after fears she was suffering from ebola, but was later given the all-clear.
Pennington said there could be another couple of cases of ebola diagnosed in the UK in the coming months.
But he added: “I am not at all concerned about any risk to the Scottish population. We have very good systems in place to stop any spread.”
Dr Devi Sridhar, a senior lecturer in global public health at Edinburgh University, said: “Cases are coming here (to the UK), but they are isolated cases.
“I worry that people will say we shouldn’t be sending volunteers over. The only way to protect the whole world is to work together.”
Meanwhile, the Scottish Government has confirmed that NHS staff in Scotland are on standby to provide help in “overspill” centres to treat additional patients if a number of cases of ebola are diagnosed in the UK at the same time.
The Royal Free – which has the UK’s only high level isolation unit – has just two beds.
A spokeswoman for the Scottish Government said: “There are plans in place to supplement this capacity with surge centres at the Royal Free, Newcastle, Sheffield and Liverpool should it be required.
“Scotland has agreed to support these units if necessary.”