Luck will play a role in the survival chances of Ebola patient Pauline Cafferkey because experts still do not know enough about the deadly virus, a microbiologist has said.

Professor Hugh Pennington said people with the disease can deteriorate "very quickly" after it emerged the British nurse is now in a critical condition at London's Royal Free Hospital.

Mrs Cafferkey, 39, has been receiving treatment at the hospital since Tuesday after contracting Ebola while volunteering in Sierra Leone.

Mr Pennington, emeritus professor of bacteriology at the University of Aberdeen, said he believed using blood from a recovered Ebola patient to treat the Scottish nurse was her "best chance" for a recovery.

"The condition of people with Ebola can deteriorate very quickly," he said.

"We have to keep our fingers crossed and hope for the best. It's the luck of the draw unfortunately.

"Some people do recover, but some don't make it. We still don't know enough about Ebola."

Mrs Cafferkey's sudden change in condition comes just days after her doctor said she was sitting up, eating, drinking and communicating with her family.

She is being treated with an experimental anti-viral drug along with the blood from a survivor after the hospital 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".

Mr Pennington said: "The plasma is probably her best chance of treatment as that is actual antibodies from people who have recovered from Ebola.

"Because of the small number of people treated with experimental drugs, it's difficult to judge the percentage of success.

"We also don't know the circumstances of the infection. That might be important."