Pauline Cafferkey, the nurse who suffered complications from the Ebola infection she contracted while volunteering in west Africa, is now critically ill, the Royal Free hospital in London has said.

The announcement shocked medical experts. While it is recognised that the virus can linger in parts of the body after a patient has recovered, it has never before been known to trigger potentially lethal disease months after the initial illness.

Jonathan Ball, a professor of molecular virology at the University of Nottingham, said it was “frankly staggering”. He said: “I am not aware from the scientific literature of a case where Ebola has been associated with what we can only assume as life-threatening complications after someone has initially recovered, and certainly not so many months after.”

Cafferkey was flown from Glasgow, where she lives, to London for specialised care in the tented isolation facility at the Royal Free after she fell ill last week. She spent nearly a month in the facility in January. Her family said the local medical staff had been slow to realise her illness was triggered by the infection she had successfully fought last year.

The hospital said on Wednesday afternoon that her condition had deteriorated and that she was critically ill. It has not released further details of her illness.

“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,” a statement said.

While there have been reports of the virus lingering in the eyes, spinal fluid and brain of survivors and of transmission through semen, Cafferkey is the first person known to have a life-threatening illness caused by a resurgence of Ebola from some reservoir in her body. It is possible other survivors in west Africa may have fallen ill in the same way without coming to public attention.

Her illness will raise questions about possible unrecognised complications among survivors in Sierra Leone, Liberia and Guinea, where healthcare services, always poor, were decimated by the outbreak.

Although 58 of Cafferkey’s contacts are being monitored, most experts believe that is a precautionary measure to reassure the public and that it is unlikely she is infectious. Ebola is transmitted only by contact with body fluids and there has been no suggestion that her new illness is manifesting in the usual way for a first-time Ebola infection, which typically involves vomiting and eventually bleeding.

On Monday, the day before she was admitted to hospital, Cafferkey was well enough to visit Mossneuk primary school in East Kilbride, where she gave a presentation to thank children for their fundraising efforts. There was no suggestion that she appeared ill while she was there.

“Whilst we don’t know her specific symptoms we have been reassured that before she was admitted to the Royal Free she wasn’t exhibiting any that we’d associate with a transmission risk to others, so contact monitoring and vaccination is a precautionary measure,” said Ball. “Without knowing the full details it is difficult to comment but I know that the team at the Royal Free will be doing everything in their powers to help her recover.”

Dr Nathalie MacDermott, a clinical research fellow at Imperial College London, agreed that any risk to the public was extremely low. “It is unlikely that anything could have been done to prevent this relapse,” she added. “This is an unexpected situation which could not have been anticipated.

“It is difficult to know whether any earlier intervention may have altered her current condition as we are only just learning about the potential long-term effects of Ebola virus disease and management of complications/secondary effects. Similarly there is still currently no proven effective treatment for Ebola virus disease that would be known to prevent or manage secondary recurrence.”

In a recent interview with ITV’s Lorraine Kelly, Cafferkey spoke of her brush with death in January. “Outwardly I just tried to be stoical about everything but inside, obviously, I was very frightened,” she said.

“I knew it could have gone three ways: it could have been mild, it could have been severe, which it was with me, and it could have been death, the other outcome which I came very close to.”

Cafferkey volunteered to work in Sierra Leone and spent four weeks there, mostly with Save the Children at its Ebola treatment centre outside Freetown. She began to feel ill when she arrived back at Heathrow airport on 28 December, where she told staff from Public Health England, who were assessing the returnees, but she was cleared to fly on to Glasgow. The following day she was admitted to the infectious diseases unit of Gartnavel hospital, Glasgow, and on 30 December she was transferred by an RAF plane to the Royal Free hospital.

On 3 January, the hospital issued a statement saying that Cafferkey was in a critical condition, but her immune system rallied, with the help of intensive medical support. On 24 January she was discharged, saying she was “happy to be alive”.