The spread of Ebola across Europe is "quite unavoidable", the World Health Organisation has warned as four people were in hospital after a Spanish nurse became the first person known to have contracted the virus outside Africa.

WHO European director Zsuzsanna Jakab has said while more cases will spread in Europe, the continent should be well prepared to control the disease.

Health officials in Spain today said four people - the nurse, her husband and two others - were being monitored in hospital in a bid to stem the spread of the virus.

"Such imported cases and similar events as have happened in Spain will happen also in the future, most likely," Ms Jakab told Reuters.

"It is quite unavoidable ... that such incidents will happen in the future because of the extensive travel both from Europe to the affected countries and the other way around," she said. Ms Jakab said European health workers were most at risk of becoming infected, but added that "the most important thing in our view is that Europe is still at low risk and that the western part of the European region particularly is the best prepared in the world to respond to viral haemorrhagic fevers including Ebola.”

It has emerged that the nurse, who had helped treat two Spanish missionaries who died after returning from Africa with the disease, first complained of feeling ill a week before she was diagnosed with Ebola on Monday.

The 40-year-old is understood to have contacted health workers after complaining of a low fever on September 30. She was only given tests for Ebola however when she turned up at hospital with a high fever on Monday, The Telegraph has reported.

One of those hospitalised is a health worker who has diarrhoea but no fever. The other is a Spaniard who travelled from Nigeria, said Rafael Perez-Santamaria, head of the Carlos III Hospital where the infected nurse treated the two Spanish missionaries.

Health officials have said 22 people who came into contact with the nurse are also being monitored. They have not been isolated but they are having their temperature taken twice a day to check for signs of infection.

The EU has now asked Spain to explain how the nurse contracted the deadly disease, according to an AFP report.

The 40-year-old nurse, who has not been identified but is said to be in a stable condition, had up to 30 colleagues who also treated the missionaries who died of Ebola at the hospital in Madrid.

A spokesman for the European Commission said the case, the first known case of Ebola spreading within a European country, would be discussed at a Health Security Committee meeting on Wednesday.

See the Ebola outbreak mapped Show all 7 1 /7 See the Ebola outbreak mapped See the Ebola outbreak mapped 25 March 2014 This outbreak of the Ebola virus first emerged in the Guéckédou region of Guinea, at a crossroads with both Liberia and Sierra Leone See the Ebola outbreak mapped 31 March On 31 March the WHO confirmed the outbreak was now international, spreading first into Liberia's northern-most Lofa region See the Ebola outbreak mapped 27 May The virus spread to Sierra Leone at the end of May - just as agencies were hoping the worst was over See the Ebola outbreak mapped 27 July In Sierra Leone the virus boomed, and then it spread to Nigeria when the Liberian diplomat Patrick Sawyer flew from Monrovia to Lagos See the Ebola outbreak mapped 9 August The Nigeria cases sparked fears around the world, and there have now been deaths in Spain and Saudi Arabia involving people who had travelled to West Africa. The numbers of cases continue to rise See the Ebola outbreak mapped 17-20 September In mid-September, Senegal confirmed its first case linked to the Ebola outbreak, a development the WHO described as a top priority emergency. Numbers of cases continued to grow exponentially in Guinea, Sierra Leone and Liberia, as experts warned they could number one million by January if not contained See the Ebola outbreak mapped 8 October Two cases of Ebola have now been reported in the US and Europe - the first times the virus has been contracted among health workers outside Africa

“The priority remains to find out what actually happened,” he said.

Officials said they were still investigating how the nurse was infected. She went on holiday after the second of the missionaries she had been caring for died on September 25, although, they stressed, she had not left Madrid.

Jonathan Ball, a professor of molecular virology at the University of Nottingham, said the Spanish nurse should not have contracted the deadly disease if appropriate containment and control measures had been taken.

“It will be crucial to find out what went wrong in this case so necessary measures can be taken to ensure it doesn't happen again,” he told Reuters.

Local media in Spain yesterday reported that staff at the Madrid hospital where the nurse became infected had claimed their protective suits did not meet health and safety requirements – though this has yet to be substantiated.

Meanwhile, a Norwegian doctor infected with Ebola while working in Sierra Leone has arrived in Norway for treatment. Officials said she was in an isolation ward at Oslo University Hospital.