For almost three weeks, doctors struggled to connect the dots between Ms. Wei and other early cases, many of them Hua’nan vendors. Patient after patient reported similar symptoms, but many, like her, visited small, poorly resourced clinics and hospitals. Some patients balked at paying for chest scans; others, including Ms. Wei, refused to be transferred to bigger facilities that were better-equipped to identify infectious diseases.

When doctors did finally establish the Hua’nan link in late December, they quarantined Ms. Wei and others like her and raised the alarm to their superiors. But they were prevented by Chinese authorities from alerting their peers, let alone the public…

Even after Chinese President Xi Jinping personally ordered officials to control the outbreak on Jan. 7, authorities kept denying it could spread between humans—something doctors had known was happening since late December—and went ahead with a Chinese Lunar New Year banquet involving tens of thousands of families in Wuhan.