Madrid, Spain – Doctors treating a Spanish nurse infected with the deadly Ebola virus were cautiously optimistic on Tuesday after she survived the potentially critical two-week period after being infected.

“She is in a stable condition and this is a good day, the 15th day” since she first showed symptoms of the illness, said Marta Arsuaga, a doctor at the specialist tropical disease hospital Carlos III.

“Statistics show that usually the disease is most deadly up to the 13th or 14th day. Now it is one day after that, so that is good,” Arsuaga told reporters at the hospital.

The infected nurse, Teresa Romero, 44, is thought to have caught Ebola in Carlos III while treating an elderly missionary who was infected in Sierra Leone and died on September 25.

She is the first person known to have caught Ebola outside Africa, where the current outbreak has killed more than 4,000 people according to the World Health Organization.

She first noticed her own symptoms on September 29 and was admitted to hospital on October 6.

Doctors say she is in a “serious but stable” condition.

“We trust this condition will be maintained,” Arsuaga said, although she cautioned that the critical phase can vary and be subject to “complications”.

The doctor said she was unable to forecast when Romero was likely to be declared out of danger.

Romero treated two elderly Spanish missionaries who were repatriated after catching Ebola. One was brought back from in Liberia in August and the second from Sierra Leone in September.

The missionaries “were clearly older and were in different conditions. This patient is young and healthy and has been here from the start of her illness. We hope her condition will develop differently,” Arsuaga said.

Sources close to Romero said she had been suffering from an accumulation of fluid in her lungs, but the doctor would not confirm this for reasons of confidentiality.

The doctor denied reports that Romero was being treated with ZMapp, one of several experimental treatments for Ebola that have been fast-tracked for development.

Fifteen people considered as having had “high-risk contact” with Romero, including her husband and two employees at a beauty salon where she went for a waxing, are being monitored at the hospital, which Spain has designated to handle Ebola cases.

None has shown any symptoms, the director of the hospital, Antonio Andreu, told a news conference.

“We are facing a situation that is perfectly under control,” he said.

“We want to be able to take all measures that prepare the health system to handle Ebola. We also want to make sure that this huge experience that Spain has can help other nations to be prepared,” he added.