The family of nurse Pauline Cafferkey have claimed doctors "missed a big opportunity" to spot she had fallen ill with Ebola again when she was told she had a virus when she attended a clinic.
Toni Cafferkey said it was "absolutely diabolical" the way her sister, who originally contracted Ebola while volunteering in Sierra Leone last year, had been treated.
Pauline Cafferkey, 39, was admitted to the Queen Elizabeth University Hospital in Glasgow on Tuesday after feeling unwell.
She has since been flown to the Royal Free Hospital in London where she is being treated in an isolation unit.
Toni Cafferkey told the Sunday Mail that her sister had gone to a GP out-of-hours clinic at the Victoria Hospital in Glasgow on Monday night but the doctor who assessed her diagnosed a virus and sent her home.
She said: "At that point me and my family believe they missed a big opportunity to give the right diagnosis and we feel she was let down. Instead of being taken into hospital, she spent the whole of Tuesday very ill.
"I think it is absolutely diabolical the way she has been treated... We don't know if the delays diagnosing Pauline have had an adverse effect on her health, but we intend to find out. It has not been good enough. We think there have been major failings and we just want her to pull through. This kind of recurrence seems to be rare but we don't yet know enough about it."
NHS Greater Glasgow and Clyde told the paper: "We can confirm that Pauline did attend the New Victoria Hospital GP out-of-hours service on Monday. Her management and the clinical decisions taken based on the symptoms she was displaying at the time were entirely appropriate.
"All appropriate infection control procedures were carried out as part of this episode of care."
Pauline Cafferkey, 39, from South Lanarkshire, was diagnosed with Ebola in December after returning to Glasgow from Sierra Leone via London.
She was critically ill and spent almost a month in an isolation unit at the Royal Free Hospital before being discharged in late January.
Ebola has been shown to persist for weeks or even months in parts of the body and in bodily fluids.
A statement from the Royal Free Hospital confirmed Ms Cafferkey had been transferred to the hospital "due to an unusual late complication of her previous infection by the Ebola virus".
It stressed: ''The Ebola virus can only be transmitted by direct contact with the blood or bodily fluids of an infected person while they are symptomatic, so the risk to the general public remains low and the NHS has well-established and practised infection control procedures in place.''