A file photo taken on December 30, 2014, shows Scottish health-care worker Pauline Cafferkey (C), who was diagnosed with Ebola after returning to Scotland from Sierra Leone. (STR/AFP/Getty Images)

Pauline Cafferkey, a nurse who contracted Ebola while doing aid work in Sierra Leone and was thought to have recovered in January, is now in critical condition due to a complication from the virus.

Cafferkey, 39, was re-admitted to the Royal Free Hospital in London on Friday after she fell ill and is being treated in an isolation unit. The hospital released a statement on Wednesday stating that, "We are sad to announce that Pauline Cafferkey's condition has deteriorated and she is now critically ill.

The severity of Cafferkey's condition underscores how much scientists still don't know about how the virus persists in the body and the dangers it can pose to the host. Ian Crozier, an American doctor also thought to have been cured of Ebola, earlier this year revealed that he had had significant vision problems and that the virus appeared to turn one of his eyes from blue to green.

[How Ebola changed an American doctor's eye color from blue to green]

An undated Cafferkey family handout photo of Scottish nurse Pauline Cafferkey.

In West Africa, doctors have reported that some Ebola survivors were also suffering all kinds of maladies, ranging from headaches to problems with eyesight. Tests have also shown that the virus appears to be present in semen, breast milk and other body fluids for months after it can no longer be detected in blood.

Close contacts of Cafferkey, including health-care workers, friends and family have been identified and 40 of the 58 were offered an experimental vaccine as a precaution, according to the Telegraph.

British health officials have said there is very little risk to the public.

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