Pay attention to small numbers: Once cases begin rising exponentially, finding them and their contacts becomes nearly impossible. “If you wait for big numbers, it will be too late,” said Paul Biddinger, an emergency preparedness expert and physician at Harvard T.H. Chan School of Public Health.

The challenge here is distinguishing between small numbers that will increase exponentially and those that won’t: two cases that become six that become 18 that become thousands, versus two cases that become six that become 18 — and then peter out.

“If you see a clear, sustained trend in case numbers from week to week,” Biddinger said, that’s likely a harbinger of exponential growth.

Determining which small numbers spell trouble is more possible if we plan now, said Amesh Adalja of Johns Hopkins University Center for Health Security, an expert on emerging infectious disease and pandemic preparedness. “With a respiratory virus that’s often asymptomatic, you know that whatever number of cases you think you have is not the number you really have; it’s only the tip of the iceberg,” he said.

Officials therefore have to take even a few cases dead seriously. But that doesn’t mean locking down a city immediately.