British nurse Pauline Cafferkey was treated in an isolation unit at London's Royal Free Hospital.

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London's Royal Free Hospital tweets that it's released Pauline Cafferkey

She was diagnosed with Ebola in late December, at one point was in critical condition

She contracted the virus after working in Sierra Leone and became ill after returning to Britain

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.

London’s Royal Free Hospital announced Saturday that “Pauline Cafferkey has been discharged from the Royal Free following her recovery from Ebola.”

On December 29, the 39-year-old Glasgow, Scotland, resident became the first person to be diagnosed with the virus on UK soil shortly after returning from Sierra Leone.

Five days later, the Royal Free Hospital announced her condition had deteriorated to critical.

That meant all the more reason to celebrate Saturday’s news.

Prime Minister David Cameron was among those weighing in, tweeting, “Great to hear Pauline Cafferkey has been discharged from hospital. Thank you to everyone @RoyalFreeNHS for the world class care.”

Worked with Save the Children in Africa

Cafferkey contracted Ebola while working with Save the Children at an Ebola treatment center, according to Michael von Bertele, humanitarian director at that organization.

British media outlets said Cafferkey is a public health nurse in Scotland’s South Lanarkshire area who was part of a 30-strong team of medical volunteers deployed to West Africa by the UK government last month in a joint endeavor with Save the Children.

She traveled via Casablanca, Morocco, and London Heathrow Airport before arriving at Glasgow Airport on a British Airways flight late last year, the health agency NHS Scotland said.

After feeling unwell, she sought medical attention and became the first person to be diagnosed with Ebola within the United Kingdom.

She was reportedly transferred to London in a military aircraft fitted with an isolation pod and brought to the Royal Free Hospital. That facility has a high-level isolation unit, with access restricted to specially trained medical staff. A specially designed tent, with controlled ventilation, is set up over the patient’s bed.

American doctor Ian Crozier was treated for Ebola in Atlanta last year and declared free of the virus in his blood.

CNN’s Susannah Cullinane and Max Foster contributed to this report.