Nurse Pauline Cafferkey was admitted to a hospital in Glasgow, Scotland
, on October 6 due to what UK health officials called "an unusual late complication" of her Ebola infection.
She was transferred to a high-level isolation unit at London's Royal Free Hospital as her condition worsened.
Dr. Michael Jacobs, a specialist on infectious diseases at the hospital, told reporters Wednesday that Cafferkey had not suffered a relapse.
"To be very clear about this, she hasn't been reinfected with Ebola virus," Jacobs said. "This is the original Ebola virus that she had many months ago which has been inside the brain, replicating at a very low level probably, and which has now re-emerged to cause this clinical illness of meningitis."
Jacobs added that doctors were treating Cafferkey with an experimental anti-viral treatment. He said doctors had discussed its use with her and that the balance of risks favored this treatment.
"We've gone through in great detail with her what we do know about the drug and what we don't know about the drug. ... This is highly experimental treatment and we don't know yet whether it's of benefit to her."
He said Cafferkey's condition had significantly improved since she was declared critically ill last week, but that she was still in the isolation unit.
"This infection is completely different. We can't say that the risk of infection is zero and this is why we are still taking precautions," he said. "She uses her iPad and eats a little."
Jacobs said doctors were very hopeful Cafferkey would make a full recovery from her illness.
More than 11,000 deaths
Cafferkey was first discharged from the Royal Free Hospital in January, having apparently overcome the virus.
The public health nurse contracted Ebola, which has killed more than 11,000 people internationally, while working as part of a 30-person team deployed by the UK government to work to contain the outbreak in Sierra Leone. She was diagnosed with Ebola shortly after returning to the UK.
A preliminary study released by the World Health Organization
this month found that the disappearance of Ebola symptoms does not mean the virus is gone.
Researchers found that Ebola can still be found in the semen of survivors for at least nine months after the onset of symptoms.
Meantime, a U.S. Ebola survivor was found to have traces of the virus in his eye
months after doctors had declared him Ebola-free.
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 Atlanta hospital that had treated him said at the time.