Spain's government has urged calm as it scrambled to identify people potentially infected by a Madrid nurse suffering from the deadly Ebola virus.
European leaders boosted emergency measures to tackle the outbreak after the nurse became the first person to contract the disease outside of Africa, raising fears of wider contagion.
Doctors in Madrid have hospitalised five other people deemed to be at risk of infection and officials said they were monitoring dozens of others.
The worst-ever outbreak of the disease has killed nearly 3500 people in west Africa since the start of the year.
Spain's Prime Minister Mariano Rajoy on Wednesday called for calm and promised "transparency" over the scare which has sparked fierce criticism of Spanish health safeguards.
The European Union has demanded answers about how the disease was able to spread inside a specialised disease unit, while Spanish health staff protested over safety failures.
The nurse was identified by Spanish media as Teresa Romero, a woman in her forties who worked at Madrid's La Paz-Carlos III hospital.
She became ill after caring for two elderly Spanish missionaries who died of Ebola following their return from west Africa.
Officials said they were trying to find out who she came into contact with before being isolated on Monday. They said they were monitoring 52 people - mostly health staff.
The nurse's husband, Javier Limon, was quoted by El Mundo newspaper as saying she had stayed "mainly at home" after feeling the first symptoms.
Doctors at the hospital said her husband was also at "high risk" and was put in isolation. Another "suspect case" - an engineer who recently travelled from Nigeria - was being monitored.
The other three patients hospitalised were medical staff from the hospital. The latest of those was admitted on Wednesday morning, the hospital said.
The infected nurse treated Spanish priest Miguel Pajares, 75, who contracted Ebola in Liberia and died on August 12, as well as Manuel Garcia Viejo, 69, who was repatriated from Sierra Leone and died on September 25.
She is believed to have caught the virus while caring for Garcia Viejo.
The European Commission has written to the Spanish health ministry demanding an explanation.
"There is obviously a problem somewhere," commission spokesman Frederic Vincent said, at a time when all EU member states are supposed to have taken measures to prevent an Ebola outbreak.
However, Peter Piot, one of the scientists who discovered Ebola, said that although the disease posed a threat to healthcare workers around the world, there was "no risk that I see for outbreaks" in developed countries.
Spanish government officials said the nurse began to feel ill on September 30 while on leave after treating the missionaries, but was not admitted to hospital until five days later.
Rajoy said the top priority was to treat the infected patient and secondly to monitor people who had been in contact with her.
Officials would meanwhile "investigate what happened and why this contagion occurred", he told parliament on Wednesday.
Health workers' unions said the nurse had called the Carlos III hospital when she felt ill, but was first referred back to a local health centre.
Doctors said she was not admitted right away because she did not yet have a high fever or other Ebola symptoms.
Health unions have complained that staff had not been adequately trained to treat Ebola patients safely.
"We do not have the infrastructure to tackle a virus like this," said Elena Moral, leader of the CSIF-AGCM union.
The EU announced it will airlift 100 tonnes of relief aid on Friday to Sierra Leone, Liberia and Guinea including masks, gloves and medicines.
Dozens of British troops are due to fly to Sierra Leone next week to help build medical facilities.
British Prime Minister David Cameron will hold an emergency meeting on Wednesday to co-ordinate his country's response to the outbreak, his office said.
The head of the US military's Africa Command, General David Rodriguez, said American troops deployed to west Africa could stay there for up to a year.
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