AFP/Madrid

Spain's government urged calm on Wednesday over the case of a nurse who caught Ebola in a Madrid hospital as the World Health Organization warned other isolated infections in Europe were "unavoidable".

Apart from the infected nurse -- the first person to contract the disease outside Africa -- doctors in Madrid have isolated five other people and are monitoring dozens more.

One of the doctors treating the nurse, Teresa Romero, said she may have caught the deadly virus after touching her face with an infected glove.

She had been treating a missionary flown back to Madrid's La Paz III hospital from west Africa with the disease.

"It seems like it was the gloves. The gloves touched the face," doctor German Ramirez told reporters outside the hospital.

The Ebola epidemic has killed nearly 3,500 people in west Africa this year and the World Bank warned on Wednesday it could cripple the region's economy to the tune of at least $32 billion (25 billion euros).

Spain was still scrambling on Wednesday to identify people who came into contact with Romero.

Prime Minister Mariano Rajoy called for calm and promised "transparency" over the scare, which has sparked fierce criticism of Spanish safeguards.

The World Health Organization also moved to calm fears of wider contagion in Europe.

Its regional director, Zsuzsanna Jakab, said sporadic cases in Europe were "unavoidable" but the risk of a full outbreak were "extremely low".

"European countries are among the best prepared in the world" to tackle such diseases, she added in a statement.

- 'The mistake' -

Romero initially told reporters she had "no idea" how she got infected.

But in an interview with El Pais newspaper, by phone from her hospital bed, she said it may have occurred when removing her protective suit.

"I think the mistake was when I removed the suit. I see it as the most critical moment in which it could have happened but I am not sure," she said.

In an earlier recording published on the website of El Mundo newspaper, she sounded tired and groggy but said she was feeling "a bit better".

Romero became ill after caring for two elderly Spanish missionaries who died of Ebola following their return from west Africa.

Doctors at the hospital put her husband and another "suspect case" -- an engineer who recently travelled from Nigeria -- in isolation.

Three other members of the hospital's medical staff were also admitted as a precaution.

Authorities have also ordered that the Romero family dog be put down as a precaution, sparking protests from animal rights activists.

- PM promises investigation -

The European Commission has written to the Spanish health ministry demanding an explanation of how the infection occurred.

Officials said they were monitoring 52 people -- mostly health staff.

They said Romero began to feel ill on September 30 while on leave after treating the missionaries, but she was not admitted to hospital until six days later.

Rajoy told parliament that officials would "investigate what happened and why this contagion occurred".

Health unions complained that staff had not been adequately trained to treat Ebola patients safely.

Some 3,439 people have died from Ebola out of 7,478 cases in the current outbreak across five west African nations -- Liberia, Guinea, Sierra Leone, Nigeria and Senegal -- according to the latest WHO tally.

The EU announced it will start airlifting 100 tonnes of relief aid on Friday to Sierra Leone, Liberia and Guinea including masks, gloves and medicine.

Ebola causes severe fever, vomiting, diarrhoea and sometimes internal and external bleeding. It spreads through contact and bodily fluids.

There is no vaccine and no widely available cure, but several treatments have been fast-tracked for development.

Several patients have been treated for Ebola in Europe and the United States in the current outbreak, but until the Madrid case all had been infected in Africa.

The United Nations mission in Liberia announced on Wednesday a second Ebola infection among its international staff there.