A Scottish nurse who contracted Ebola while working in Sierra Leone in Africa is in critical condition despite treatment with experimental anti-viral drugs and blood donations from survivors of the disease, doctors say.

Pauline Cafferkey was diagnosed with the deadly virus in December after returning from working for a month with the Save the Children charity in Sierra Leone.

The Royal Free Hospital reported it was "sorry to announce that the condition of Pauline Cafferkey has gradually deteriorated over the past two days."

Cafferkey, 39, had returned to home by way of Casablanca, Morocco, arriving at London's Heathrow Airport, where she underwent screening for Ebola after telling health officials she thought she might be coming down with a fever.

When her temperature was recorded as normal in seven measurements over 30 minutes, she was given permission to fly on to her home in Scotland.

She was subsequently admitted to a hospital in Glasgow after a fever developed, then flown to the Royal Free Hospital's special treatment center in London.

Seventy-one passengers on the flight Cafferkey took from London to Glasgow have been contacted about screening by Public Health England, a PHE spokeswoman said.

Passengers on the Casablanca to Heathrow flight have also been contacted.

Cafferkey is the second United Kingdom case of Ebola; another nurse, William Pooley, recovered from the disease at the Royal Free hospital in September.

He has since returned to West Africa to help treat those battling Ebola.

Earlier in the week, doctors at the London hospital had expressed cautious optimism about Cafferkey's recovery progress before her condition began to worsen.

"We are giving her the very best care possible," said infectious disease consultant Dr. Michael Jacobs. "However, the next few days will be crucial. The disease has a variable course and we will know much more in a week's time."

Experts say patients can have different responses to Ebola treatment.

David Mabey, an expert in communicable diseases at the London School of Hygiene and Tropical Medicine, said Cafferkey's initial reaction to the Ebola virus would have been difficult to predict.

"The critical period is in the first four or five days after it's diagnosed, because, you know, if you are going to get worse then that's when it happens, and I'm very sorry to hear that seems to have been the case," he said.

There have been more than 20,200 reported cases of Ebola since the outbreak began in West Africa in March of last year, with the death toll approaching 7,800 people, according to the World Health Organization.