Story highlights Scottish nurse Pauline Cafferkey is in serious condition at a hospital isolation unit

Health officials say she has an "unusual late complication" from an earlier Ebola infection

She was initially treated for Ebola in December but was discharged the following month

London (CNN) The first person diagnosed with Ebola in the United Kingdom is back in the hospital with a complication from the virus, and doctors say her condition is serious.

Pauline Cafferkey, who was initially discharged from a hospital in January after recovering from the virus, is being treated in a high-level isolation unit at London's Royal Free Hospital.

The hospital said in a statement Friday that the 39-year-old nurse was in serious condition.

It said that Cafferkey had been admitted early Friday "due to an unusual late complication of her previous infection by the Ebola virus."

She had been transferred there from Queen Elizabeth University Hospital in Glasgow, where she had been treated in the infectious diseases unit since Tuesday after feeling unwell, according to a statement from the National Health Service, or NHS.