A British nurse who contracted Ebola in Sierra Leone and was hospitalized in London this week is now in "critical" condition, the hospital treating her said Saturday.
"The condition of Pauline Cafferkey has gradually deteriorated over the past two days and is now critical," the Royal Free Hospital in London said in a statement.
On Wednesday, doctors had said the 39-year-old Scot was sitting up in bed, reading and talking to staff from inside her isolation tent in the hospital.
They said Cafferkey, who was working with the charity Save the Children in Sierra Leone, had agreed to have blood plasma treatment and take an experimental anti-viral drug.
However, they were not able to give her ZMapp, the drug successfully used to treat fellow British volunteer nurse William Pooley, because global supplies had run out.
The plasma was taken from the blood of a patient successfully treated in Europe, in the hope that the antibodies it contained would help her fight the virus.
Doctor Michael Jacobs, infectious diseases consultant at the Royal Free, had warned at the time that "Ebola runs a very variable course and the next few days are going to be very critical".
Cafferkey, who works for the state-run National Health Service in Scotland, had been volunteering at a British-built treatment center in Kerry Town when she contracted the deadly virus.
British Prime Minister David Cameron said on Twitter: "My thoughts and prayers are with nurse Pauline Cafferkey who is in a critical condition with Ebola."
Health Secretary Jeremy Hunt added that doctors were "working tirelessly to provide her with the best possible care."
Cafferkey is the second person to be treated for Ebola in Britain after Pooley, who recovered and has since returned to Sierra Leone.
She was diagnosed in Glasgow on Dec. 29 after flying home, and was transferred to the Royal Free, which has the only isolation ward in Britain equipped for Ebola patients.
Meanwhile in Swindon, southwest England, a person with a history of travel to west Africa was being tested for Ebola as a precautionary measure after being admitted to a hospital.
Around 100 people have been tested for Ebola in hospitals across England during the current outbreak, with all of them testing negative so far apart from Cafferkey.
British health authorities said Friday they had traced all the British-based passengers who traveled on the last two legs of Cafferkey's journey to Scotland.
She flew from Sierra Leone's capital Freetown to Casablanca in Morocco, and then on to London Heathrow Airport and finally Glasgow.
Her temperature was taken at Heathrow but did not raise alarms, and she was cleared to fly on to Glasgow.
Ebola has killed 7,890 people in the past year, out of 20,171 cases, according to the latest tally by the World Health Organization.
Almost all the deaths and cases have been recorded in the three west African countries worst hit by the outbreak: Sierra Leone, Liberia and Guinea.
It is spread only through direct contact with the bodily fluids of an infected person showing symptoms such as fever or vomiting, and as such people caring for the sick are particularly exposed.
As of Dec. 28, not counting Cafferkey, a total of 678 healthcare workers were known to have contracted the virus, and 382 of them had died, the WHO said.