MADRID — The Spanish health authorities said Thursday that the condition of an auxiliary nurse infected by Ebola had worsened, three days after she became the first person to test positive for the disease in Europe.

The deterioration in the nurse’s condition came as the authorities announced that one more health care worker had been quarantined, in addition to three others who were isolated overnight at the same hospital where the nurse works.

Altogether, about 80 people are being monitored to see if they develop symptoms of Ebola as Spain seeks to prevent the virus from spreading. Seven people are now quarantined at the Madrid hospital, Carlos III, that Spain has designated to handle Ebola cases.

Yolanda Fuentes, the hospital’s deputy director, told reporters on Thursday that the condition of the infected nurse, María Teresa Romero Ramos, had worsened, without providing details. The brother of the nurse told a regional television station that he had been advised by one of the doctors treating his sister that there was “not much hope” that she could overcome the virus.