A Spanish nurse who is the first person known to have been infected with Ebola outside Africa is at “serious risk” of dying after her condition worsened Thursday, officials said.

A spokeswoman for the La Paz-Carlos III hospital where Romero is being treated told reporters: “Her clinical situation has deteriorated but I can’t provide more information,” on the patient’s request.

Romero helped treat two elderly Spanish missionaries who died after returning from west Africa with Ebola in August and September. She tested positive for the disease on Monday.

Her case has heightened concerns that the worst epidemic of Ebola on record could spread from west Africa, where it began late last year. It has since killed nearly 3,900 people, mostly in Liberia, Sierra Leone and Guinea.