For more than a year, China has held SARS-CoV-2 at bay, suffering but a few cases at a time in controllable outbreaks. All of the infections were carried into China from the outside, often by Chinese returning from foreign travel. All visitors arriving in China face two weeks of quarantine — but those rules may have been changing as China's vaccination saturation inched toward the top.

Whenever new infections popped up, or were caught in massive testing sweeps, contract tracers immediately descended on the patient. His path was meticulously retraced using cell-phone location tracking and relevant survellience footage. Face recognition identified everyone the patient encountered or interacted with. In every case, a large sample of mostly strangers were interviewed, tested, and potentially quarantined. The strategy is very aggressive, but it has allowed the rest of China to mingle and socialize and go about their business.

But in Nanjing, the Coronavirus slipped in through an unexpected weakness in their maintenance procedures.