Investigations are under way at a hospital in Madrid after a Spanish nurse became the first person known to have contracted the deadly Ebola virus outside West Africa.
The nurse had treated two Spanish missionaries who died of the disease after being flown home from the region.
Three other people, including the nurse's husband, have been quarantined.
The European Commission has asked Spain to explain how the nurse could have become infected.
Some 3,400 people have died in the outbreak - mostly in West Africa.
The Spanish auxiliary nurse, a 40-year-old woman who has not been named, was one of about 30 staff at the Carlos III hospital in Madrid who had been treating priests Manuel Garcia Viejo and Miguel Pajares, officials say.
Mr Garcia Viejo, 69, died at the hospital on 25 September after catching Ebola in Sierra Leone. Mr Pajares, 75, died in August after contracting the virus in Liberia.
The nurse had twice gone into the room where Mr Garcia Viejo had been treated, to be directly involved in his care and to disinfect the room after his death. Both times she was wearing protective clothing.
Madrid healthcare director Antonia Alemany told reporters that according to the information available: "The nurse went into the room wearing the individual protection gear and there's no knowledge of an accidental exposure to risk."
Shortly afterwards the nurse went on holiday, a hospital spokesman said, but fell ill on 30 September and was admitted to Alcorcon hospital in south-west Madrid on Sunday after being tested positive for Ebola.
Early on Tuesday she was moved under police escort to Carlos III hospital in the capital and is said to be in a stable condition.
The Spanish health authorities say she is being treated with a drip using antibodies from previous Ebola patients.
Her husband and a second nurse who treated the missionary are now in quarantine, officials said, as well as a man who recently arrived on a flight from Nigeria.
- Avoid direct contact with sick patients
- Wear goggles to protect eyes
- Clothing and clinical waste should be incinerated and any medical equipment that needs to be kept should be decontaminated
- People who recover from Ebola should abstain from sex or use condoms for three months
Doctors are also monitoring 22 people who the nurse had contact with at Alcorcon hospital, and 30 people working at Carlos III, according to health sources quoted by Spanish newspaper El Pais.
They include an ambulance crew, and doctors and nurses, and have all been contacted by the health authorities.
It was not clear where the nurse had gone on holiday.
It is also unclear how she could have contracted Ebola.
The hospital was reported to have had extreme protective measures in place including two sets of overalls, gloves and goggles.
However, health workers told El Pais newspaper that the clothing did not have level-four biological security, which is fully waterproof and with independent breathing apparatus.
Instead it was level two, the paper says, as photographs provided by staff indicated that the overalls did not allow for ventilation and the gloves were made of latex and bound with adhesive tape.
Containing Ebola: BBC health correspondent Michelle Roberts
Patients should be isolated - ideally in a hospital with the highest level of bio-safety.
Such a facility would use a specially designed tent with controlled ventilation to house the patient's bed - this allows staff to provide clinical care while containing the infection.
Staff treating the patient must wear protective suits, gloves, masks and goggles. This equipment should be completely impermeable since Ebola is spread in bodily fluids such as sweat, urine and blood
Any clinical waste such as syringes, paper towels or clothing from the patient should be incinerated. A dedicated laboratory should be used to carry out any necessary tests.
European Commission spokesman Frederic Vincent said a letter had been sent to the Spanish health minister "to obtain some clarification" about how the nurse had contracted Ebola despite all the precautions taken.
"There is obviously a problem somewhere," he said.
In other developments:
- British health authorities have said there are no plans to introduce Ebola screening for travellers arriving in the UK
- A container of aid materials designed to help fight the Ebola outbreak but stranded at a port is at the centre of a political row in Sierra Leone
- A Norwegian medic who tested positive for Ebola while working with Medecins Sans Frontieres in Sierra Leone has been flown back to Oslo
- Shares in airlines including Easyjet and British Airways owner IAG have fallen sharply following the news of the Ebola case in Madrid
Ebola spreads through contact with bodily fluids and the only way to stop an outbreak is to isolate those who are infected.
There have been nearly 7,500 confirmed infections worldwide, with officials saying the figure is likely to be much higher in reality.
Guinea, Sierra Leone and Liberia have been hardest hit.
Thomas Duncan, the first person to be diagnosed with Ebola in the US, is being treated at a Dallas hospital. He caught the virus in his native Liberia. Doctors say he is critical but stable.
Ebola virus disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Spread by body fluids, such as blood and saliva
- Fatality rate can reach 90% - but current outbreak has mortality rate of about 70%
- Incubation period is two to 21 days
- There is no proven vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats, a delicacy for some West Africans, are considered to be virus's natural host