Image caption Pauline Cafferkey works as an associate public health nurse at Blantyre Health Centre, South Lanarkshire

A British nurse who was diagnosed with Ebola after returning from Sierra Leone is now in a critical condition, the London hospital treating her has said.

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

Ms Cafferkey, from South Lanarkshire, was given an experimental anti-viral drug and blood from disease survivors.

Meanwhile, a patient who was tested in Swindon for Ebola has tested negative.

'Best possible care'

Ms Cafferkey, a public health nurse, was diagnosed with Ebola in December after volunteering with Save the Children in Sierra Leone.

On Saturday 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 also expressed his concern, adding: "I know Dr Mike Jacobs and his team at the Royal Free Hospital are working tirelessly to provide her with the best possible care."

Scotland's First Minister Nicola Sturgeon tweeted: "My thoughts are with Pauline & her family at this extremely difficult time. Thanks to all who are caring for her."

Ms Cafferkey had travelled home via Casablanca, Morocco, and London's Heathrow Airport.

She was screened for the disease at Heathrow where she told officials she believed a fever might be developing.

Her temperature was taken seven times in total, six of which were within 30 minutes, and was normal each time, so she was allowed to fly home to Scotland.

The government's chief medical officer, Dame Sally Davies, has said the case raises questions about airport screening procedures.

Image copyright Other Image caption Ms Cafferkey was part of a group of up to 50 NHS healthcare workers who volunteered in Sierra Leone

Ms Cafferkey was later placed in an isolation unit at Glasgow's Gartnavel Hospital after becoming feverish, before being transferred by RAF Hercules plane to London and on to the Royal Free's specialist treatment centre.

Officials from Health Protection Scotland have spoken to all 71 people aboard the British Airways flight from Heathrow to Glasgow that Ms Cafferkey took - a Public Health England (PHE) spokeswoman has said.

And all 101 UK-based passengers and crew aboard the Royal Air Maroc flight from Casablanca to Heathrow have been contacted by PHE officials.

The remaining 31 international passengers on the flight were being traced by international health authorities, the spokeswoman added.

Dr Nick Beeching, an infectious disease specialist and a senior lecturer at the Liverpool School of Tropical Medicine, told the BBC the risk to the general public of disease spreading was "almost nil".

He said Ms Cafferkey was receiving the best possible care, and the doctors and nurses at the Royal Free were aware of the risks of contamination.

'Critical period'

Ms Cafferkey's is the second UK case of Ebola. Another nurse - William Pooley - recovered from Ebola in September after also being treated at the Royal Free Hospital.

He donated some blood plasma and was treated with the anti-viral drug ZMapp, of which there are no stocks left.

Microbiologist Professor Hugh Pennington said patients responded to Ebola treatment differently.

"Some patients with Ebola get sick and then they get better. Not everybody dies," he said.

For this reason, he said, it was "very difficult" to tell how effective treatments would be - especially when "relatively small numbers of people are being treated with these various experimental approaches".

'Critical period'

David Mabey, an expert in communicable diseases from the London School of Hygiene and Tropical Medicine, also said Mrs Cafferkey's reaction to the virus would have been hard to predict.

"A proportion of people don't get severely ill; Will Pooley was an example - he was never very sick and he recovered fully within a few days.

"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."

Dr Chris Smith, a consultant virologist at Cambridge University, said symptoms usually develop "abruptly" and peak after "about seven days".

After 10 days, he added: "Usually they've turned the corner and they begin to improve."

Ebola is transmitted by direct contact with the bodily fluids of an infected person, such as blood, vomit or faeces.

The virus has killed more than 7,800 people, almost all in West Africa, since it broke out a year ago.

The World Health Organization says the number of people infected by the disease in Sierra Leone, Liberia and Guinea has now passed 20,000.