Teresa Romero Ramos, the nurse assistant who tested positive for Ebola in Madrid, is in critical condition, a hospital official has warned.
The assistant director of Madrid's La Paz-Carlos III hospital, where Romero worked and is being treated, said the woman's health has deteriorated.
"Her clinical situation has deteriorated but I can't give any more information due to the express wishes of the patient," Yolanda Fuentes said.
Earlier, Madrid's regional president Ignacio Gonzalez told parliament the nurse "was at this time very ill and her life is at serious risk as a consequence of the virus".
The 44-year-old woman, who is in quarantine with her husband Javier Limon Romero, assisted two missionaries who died from Ebola after returning from Sierra Leone in West Africa.
According to the latest developments, she contracted the virus when she touched her face with the gloves she used to attend one of the two missionaries.
She was the first person believed to have contracted the virus outside west Africa, where the disease has killed more than 3,800 people since last January.
Reports emerged on Tuesday 7 October that two other people suspected of being infected were admitted to hospital in Spain.
The woman's dog, Excalibur, was put down on suspicions he had contracted the virus as well.
The euthanisation sparked protests in Madrid, with dozens of people taking to the streets to impede officials attempting to remove the pet from Romero's house.
Ebola is spread via contact with infected bodily fluids. WHO has assured people that the deadly disease is not airborne, following recent speculation.
Tom Frieden, director of the Centres for Disease Control, warned that unless action is taken, Ebola could become a crisis to rival Aids.
"In the 30 years I've been working in public health, the only thing like this has been Aids," he said at a World Bank and International Monetary Fund annual meeting in Washington