A company that uses internet-connected thermometers to predict the spread of the flu says it is tracking the coronavirus in real time — something that had been impossible, given the lack of testing for the disease.

Kinsa Health has sold or given away more than a million smart thermometers to households in which two million people reside, and thus can record fevers almost as soon as consumers experience them.

For the last few years, Kinsa’s interactive maps have accurately predicted the spread of flu around the United States about two weeks before the Centers for Disease Control and Prevention’s own surveillance tool, the weekly FluView tracker.

The thermometer data “acts as an early warning system for illness spreading,” said Inder Singh, the company’s founder. The C.D.C.’s system lags because it relies on weekly reports from hundreds of doctors’ offices and hospital emergency rooms about what symptoms they are seeing in patients.