Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

LONDON — A British nurse who contracted Ebola was flown back to a secure hospital unit Friday after suffering "an unusual late complication of her illness," health officials said.

Pauline Cafferkey was discharged from London's specialist Royal Free Hospital in January after contracting the virus while working with a charity in Sierra Leone.

Scottish healthcare worker Pauline Cafferkey at the Royal Free Hospital in London on January 24. LISA FERGUSON / SCOTLAND ON SUND / EPA

Cafferkey, who is from South Lanarkshire, Scotland, was flown by military aircraft under the supervision of experts back to the specialist unit in the early hours of Friday, Public Health England said in a statement.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

A spokesman for Scotland's NHS Greater Glasgow and Clyde health authority confirmed to NBC News on Friday that Ebola had been detected in Cafferkey, but because the nurse had suffered the virus before this was "not a new infection."

Cafferkey was discharged from hospital more than nine months ago. But she was admitted to Glasgow's Queen Elizabeth University Hospital again Tuesday after reporting feeling unwell, NHSGGC said in statement.

She was treated in the infectious diseases unit before being flown to the Royal Free Hospital, which houses the U.K.'s only high-level isolation unit, in the early hours of Friday morning following the unspecified "complication."

Dr Emilia Crighton, director of public health at NHSGGC, said the risk to the public was "very low," but she added that officials had identified a number of people with whom Cafferkey may have come into contact and would be "following up as a precaution."

The news came just two days after the World Health Organization announced that the three West African countries at the heart of the epidemic had their first Ebola-free week since March 2014.

More than 11,000 people died and some 28,000 have been infected across Guinea, Liberia and Sierra Leone in the worst known outbreak of the disease.

Eleven patients were treated in the United States, mainly medical missionaries and medical volunteers. Two people died from the disease in the U.S.