A Spanish nurse who treated two Ebola patients at a Madrid hospital is thought to be the first person to have contracted the virus outside Africa.
"She is a health professional who took care of the infected with the disease who were repatriated and cared for at Carlos III hospital," the director of Spain's public health department, Mercedes Vinuesa, told a media conference.
Health Minister Ana Mato also told the news conference that an emergency protocol had been put in place and authorities were working to establish the source of the contagion.
Both the patients the nurse helped care for died from the disease.
Spanish priest Miguel Pajares, 75, was infected with Ebola in Liberia and died at Madrid's La Paz-Carlos III hospital on August 12.
Another Spanish missionary, Manuel Garcia Viejo, 69, was repatriated from Sierra Leone and died at the same hospital on September 25.
Both were members of the Hospital Order of San Juan de Dios, a Catholic group that runs a charity working with Ebola victims in Africa.
Spanish health officials said they were monitoring 30 other health workers who attended the priests.
The assistant nurse was admitted to hospital on Monday morning with a high fever, Spanish newspaper El Pais reported.
Doctors then isolated the emergency treatment room, the report said.
The deadly Ebola haemorrhagic fever virus has killed more than 3,400 people since it began in West Africa in March and has now started to spread faster, infecting almost 7,200 people so far.
In recent weeks it has spread to Nigeria, Senegal and the United States - where the first case was diagnosed last week - by unwitting travellers carrying the virus.
US calls for help for West Africa
In the United States, president Barack Obama said he planned to step up pressure on large countries to contribute aid to West African nations struggling to contain the worst outbreak of Ebola on record.
He said the chance was "extremely low" but there was not a large margin for error.
"The good news is that it's not an airborne disease - we know what has to be done and we have the medical infrastructure to do it," he said.
"But this is an extraordinarily virulent disease when you don't follow the best protocols."
Mr Obama said the US was not currently considering a travel ban from West Africa but officials were working on new protocols to screen airline passengers to identify people who might have Ebola.
He said the US would increase efforts to make medical professionals aware of what to do if they encountered a case.
I am going to put a lot of pressure on my fellow heads of states. Some large countries are not doing enough.
Barack Obama, US President
Meanwhile, the first Ebola patient diagnosed in the United States is now receiving an experimental drug for the disease.
A spokeswoman for Texas Health Presbyterian Hospital in Dallas said he was receiving the drug, called Brincidofovir, developed by Chimerix Inc.
The patient, Thomas Eric Duncan, remains in critical but stable condition, the hospital said.
Questions have been raised about why Mr Duncan had not received experimental therapies in the week since he was admitted to hospital.
Experimental drugs had been given to at least three other Ebola patients flown to the US from West Africa.
All three recovered but it is not known whether the drugs played a role in their recovery.
At a briefing on Sunday, US Centers for Disease Control and Prevention director Dr Thomas Frieden said there were no more doses available of the drug ZMapp, which was used to treat two of the patients.
The third patient was treated with TKM-Ebola which Dr Frieden said could be difficult for patients to take and could make them worse.
Chimerix said Brincidofovir had been tested in more than 1,000 patients without raising safety concerns.
An American cameraman diagnosed with Ebola in Liberia has arrived at a hospital in Nebraska for treatment in an isolation unit.
Ashoka Mukpo, 33, arrived in the US on a specially-equipped plane and was attended by doctors in special suits. He was able to walk unaided.
His father Michael Levy said he saw his son from a distance.
"He looked strong, he waved. He was tentative and frightened but I think he's strong and his symptoms are not more advanced," Mr Levy said.