By Emma Pinedo and Sarah Morris

MADRID (Reuters) - A Spanish nurse has become the first person to contract Ebola outside of Africa, casting doubt over measures taken in Spain to control the potential spread of the deadly disease.

The nurse had helped to treat two priests who contracted Ebola in Africa and were repatriated to Spain. Some 30 other health workers and those who came in contact with her are now being monitored for symptoms.

Both priests died shortly after reaching Spain. Each had worked in West Africa, where an epidemic of Ebola has spread through Guinea, Sierra Leone and Liberia since March, killing more than 3,400 people in the largest outbreak of the disease in history. Cases have also reached Senegal and Nigeria.

Spanish officials said they still had to find out how the nurse, who was not named but identified as married with no children, contracted the viral infection, which causes fever and bleeding.

"At the moment we are investigating the way in which the professional was infected," Antonio Alemany, the head of Madrid's primary health care services told a news conference.

The nurse was one of a specialist team who treated elderly priest Manuel Garcia Viejo at the Madrid hospital Carlos III when he was repatriated from Sierra Leone with Ebola on Sept 21. He died four days later.

Garcia Viejo was kept in isolation during his treatment last month and officials said they followed a strict protocol designed to protect health workers and patients at the hospital.

The nurse who has since fallen ill only entered Garcia Viejo's room twice, once after his death, Alemany said.

Health authorities said she had also helped treat Miguel Pajares, who had been working in Liberia when he came down with the disease. He was airlifted back to Spain on Aug 7 and died five days later.

ON HOLIDAY

The Spanish nurse went on holiday immediately after Garcia's death on Sept 25 and began feeling sick on Sept 30, said Alemany. He did not say where the nurse went on holiday.

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"We have started studying all of the contacts the patient had since her symptoms began, including the health professionals who have been treating her," Alemany said. The nurse's husband was also being monitored, he said.

Scientists tracking the Ebola epidemic in West Africa and analyzing air traffic data have predicted a high risk of a case being imported unwittingly into Europe before the end of this month.

The first Ebola case to be diagnosed in the United States was identified last week in a man from Liberia, who U.S. health officials say is now in a critical condition.

The Geneva-based WHO said it was notified of the Spanish case at around 3:00 p.m. EDT on Monday.

"This was a preliminary notification and Spain is doing an intensive investigation into the mode of transmission and into the contacts of the nurse," a WHO spokeswoman said.

Patients are at their most contagious when Ebola is in its terminal stages, inducing both internal and external bleeding, and profuse vomiting and diarrhea - all of which contain high concentrations of infectious virus.

But the disease can also have a long incubation period - up to 21 days - meaning that people can be unaware for weeks that they are infected, and not feel or display any symptoms.

Virologist Benjamin Neuman of the Britain's University of Reading said health workers always face risks handling Ebola patients regardless of whether they used protective equipment.

"Nurses face a problem in that a person who is sick with Ebola can make quite a lot of highly infectious waste, as the patient loses fluid through diarrhea and vomiting," he said.

"Those bodily fluids can contain millions of Ebola viruses, and it only takes one to transfer the infection."

The nurse, who was initially treated for fever symptoms at the hospital of Alcorcon, in the outskirts of Madrid, was due to be transferred later on Monday to the Madrid hospital Carlos III where Garcia Viejo was treated.

She is in a stable condition, health officials said.

(Additional reporting by Carlos Ruano and Sarah White in Madrid and Kate Kelland in London; Editing by Julien Toyer, Larry King)