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.
Dr. Emilia Crighton, director of public health for NHS Greater Glasgow and Clyde, said in a statement that the risk to the public from Cafferkey's condition was "very low."
"In line with normal procedures in cases such as this, we have identified a small number of close contacts of Pauline's that we will be following up as a precaution," she said.
The virus can be transmitted only by direct contact with the blood or bodily fluids of an infected person.
Worked in Sierra Leone
In December, the Glasgow resident became the first person to be diagnosed with the virus on UK soil shortly after returning from Sierra Leone.
Cafferkey contracted the virus while working with Save the Children at an Ebola treatment center, said Michael von Bertele, humanitarian director at that organization.
Five days later, the Royal Free Hospital announced her condition had deteriorated to critical. But in January, she was discharged
, with British Prime Minister David Cameron among those tweeting his congratulations.
Ebola in 'recovered' patient's eye
Cafferkey is not the first medical professional to have problems after being diagnosed as Ebola-free.
American doctor Ian Crozier was treated for Ebola in Atlanta last year and declared free of the virus in his blood.
But less than two months after being discharged, Crozier started experiencing problems with his vision, and doctors were stunned to find traces of the virus in fluid from his eye.
Despite the presence of the virus, samples from tears and the outer eye membrane tested negative, which meant the patient was not at risk of spreading the disease during casual contact, the hospital said at the time.
Crozier received steroids and an antiviral agent, and his eye gradually returned to normal. His case prompted a warning to doctors to watch Ebola survivors for eye problems.
According to the World Health Organization
, there have been 11,297 deaths, and 28,421 confirmed, probable and suspected cases in Guinea, Sierra Leone and Liberia
, the most affected countries, since the outbreak began.
The spread of the disease has slowed, with no confirmed cases of Ebola reported in most affected countries in the week to October 4 -- the first time that had happened since March 2014, according to the WHO.