Ebola arrived in the streets of America just after seven o’clock on the evening of Saturday, September 20, 2014, when United Airlines Flight 822 touched down at Dallas/Fort Worth International Airport. The deadly virus was coursing through the bloodstream of a single passenger, a 42-year-old Liberian man named Thomas Eric Duncan, a chauffeur who had flown via Brussels and Washington, D.C., with plans to marry the mother of his 19-year-old son, a 54-year-old Dallas woman named Louise Troh.

No one knew he had it, including Duncan himself, though he may have suspected. He had lied on his departure forms in Liberia, failing to mention the young Ebola victim, his landlord’s daughter, he had helped carry to a hospital. In Dallas, Duncan headed to Troh’s apartment, at the Ivy Apartments, a tidy warren in the city’s Vickery Meadow neighborhood. In recent years Vickery Meadow—while only a few miles from some of the city’s wealthiest neighborhoods, where Mark Cuban and Ross Perot and members of the oil-rich Hunt family live—has emerged as a haven for all manner of refugees and immigrants. Walking into the Ivy complex is like walking into the United Nations; women in brightly colored headscarves linger outside apartments that house families from countries as far afield as Kazakhstan, Bhutan, and Myanmar.

Five days later, on Thursday, September 25, Duncan began feeling unwell. He went to the emergency room at Texas Health Presbyterian Hospital, known locally as “Presby”—one of the city’s largest medical facilities—where he complained of a headache and nausea. A doctor, failing to notice he was also running a fever that spiked at 103 degrees and that he had recently traveled from Africa, never imagined he might have Ebola, gave him a prescription for antibiotics, and sent him home. Over the next two days Duncan began to sweat profusely and took to his bed, his fever rising. He was returned to Presby by ambulance Sunday afternoon. Despite widespread reports to the contrary in following days, he had yet to show other signs of Ebola, including excessive vomiting and diarrhea. This time, though, a nurse, taking note of his origins, decided to make a call.

Dallas County employs a squad of nine epidemiologists—they call themselves “epis”—headed by a sharp young doctor named Wendy Chung. Among their duties is manning a 24-hour phone line that area doctors and nurses can call if they suspect a new infectious disease. That afternoon one of Chung’s epis, 30-year-old Sonya Marie Hughes, was gardening with her 5-year-old at their home, near Love Field, when the phone rang. Hustling into the house, Hughes spoke to the answering service, which told her they had a possible Ebola patient at Presby.

The epis call such a patient a potential “person under investigation,” or P.U.I. With Ebola raging in West Africa, the county had already had eight potential Ebola P.U.I.’s, all false alarms, so the call didn’t especially worry Hughes; the patient was saying he had never been exposed to the virus. Hughes talked to the attending nurse, took down the information, and alerted Chung, who by late afternoon was studying Duncan’s blood work, details of which were faxed to her home. Chung didn’t like what she saw. The blood was low on platelets, something Ebola might cause. While there was still no vomiting or diarrhea, Chung decided to take no chances. She wanted more tests.

In calls to the Centers for Disease Control and Prevention, in Atlanta, that evening, however, Chung was surprised when her counterparts there saw no need for them. “The C.D.C. was actually not real keen on testing; there was no indication he’d been to a funeral, no evidence he’d eaten bushmeat, no evidence of Ebola exposure,” she says. “All he was was a gentleman from Liberia with a fever.”