A British nurse who was successfully treated in January after contracting Ebola in Sierra Leone is now "critically ill" due to a resurgence of the virus, the hospital treating her said on Wednesday.

   "We are sad to announce that Pauline Cafferkey's condition has deteriorated and she is now critically ill. Pauline is being treated for Ebola in the high-level isolation unit at the Royal Free Hospital," it said in a statement.

   Cafferkey was taken to the specialist unit in London -- the only one of its kind in Britain -- last week after being admitted to a hospital in Glasgow on Tuesday when she felt unwell.

   Her sister Toni has said doctors diagnosed her with a virus on Monday but had sent her home, missing an opportunity to act quickly.

   Health officials have contacted and are monitoring 58 people who have been in close contact with Cafferkey, 40 of whom have had direct contact with her bodily fluids -- the main mode of Ebola transmission.

   Of the 40, 25 are receiving a vaccine and a further 15 have either declined it or were unable to receive it due to existing medical conditions.

   Cafferkey was diagnosed with Ebola in December after returning to Glasgow from Sierra Leone.

   She spent almost a month in the isolation unit at the Royal Free Hospital and was treated with an experimental anti-viral drug and blood from survivors of the Ebola disease.

   Details of her condition have not been disclosed for reasons of patient confidentiality.

   Doctor Ben Neuman, a lecturer in virology at the University of Reading, said earlier that Cafferkey could be only the second known case of "reactivated" Ebola.

"Over the past few years, there has been mounting evidence of the mental and physical problems in Ebola survivors that can last for years after the virus is cleared from the bloodstream," he told the Science Media Centre in London.

   "The newly-discovered twist on this post-Ebola syndrome is that, in some cases, the health problems -- often including damage to the eyes and joints -- is actually caused by live Ebola virus growing in bodily fluids in some of the less-accessible compartments of the body."

   The deadliest-ever Ebola outbreak since the virus was identified in central Africa in 1976 has killed 11,312 of the 28,457 people infected since December of 2013, according to the latest WHO figures.

   Nearly all the victims have been in Guinea, Liberia and Sierra Leone. (AFP)