Save the Children says no stone will be left unturned in its probe as to how a Scottish nurse who volunteered at the charity’s Ebola treatment centre in Kerry Town, Sierra Leone contracted the virus.

Over the weekend, the Royal Free hospital in London issued a statement that “the condition of Pauline Cafferkey has gradually deteriorated over the past two days and is now critical". The news came as a surprise given that her doctor said she had been eating, drinking and communicating with her family on New Year’s Day.

Save the Children’s Rob MacGillivray told the BBC that “because of this very serious event we have put in an extraordinary review to ensure that we do everything can leave no stone unturned to, as far as possible, identify the source of this infection”. He added that “everybody is exposed to a certain amount of risk working in Sierra Leone at the moment but we will certainly be focusing on how the personal protection equipment was used, how it was put on, and more importantly how it was taken off”.

Ms Cafferkey’s doctor Michael Jacobs said she faced a critical few days while she is treated with convalescent plasma taken from the blood of an Ebola survivor and an unnamed experimental antiviral. It is not ZMapp, which was used to treat William Pooley, the first Briton who contracted Ebola and got the all-clear from the Royal Free in September, because “there is none in the world at the moment”, Dr Jacobs said.