Cafferkey contracted Ebola in Sierra Leone in 2014 and spent just under a month in London’s Royal Free Hospital.
The 39-year-old nurse was readmitted in the early hours of Friday morning, 10 months after she first recovered.
It is understood that she is in a ‘serious condition’, which is an ‘unusual late complication’ of a previous infection with the virus.
Her isolation is a precaution as head officials say she is not contagious and poses a ‘low risk’.
Tests have revealed that the Ebola virus is still present in her body.