DALLAS — A biohazard barrel sat on the front lawn, and red caution tape was looped around tree trunks. Police officers stood guard and helicopters whirred overhead as decontamination crews cleaned off an apartment building home to a hospital worker who had tested positive for Ebola.

For residents in a city that has become the ground zero of Ebola in the United States, what has become an unnervingly familiar scene unfolded Wednesday morning at an apartment complex north of downtown Dallas in the hours after officials announced that a second medical worker who treated a Liberian man who died of Ebola had tested positive for the virus.

The worker, identified as Amber Joy Vinson, 29, is an Ohio native who lived in a ground-floor apartment in a sprawling complex of 330 apartments that serves as a first stop for many young professionals in the area. She was one of dozens of doctors, nurses and others who helped care for Thomas Eric Duncan at Texas Health Presbyterian Hospital here. She reported a fever on Tuesday and was immediately isolated at the hospital.