As governments around the world have pushed their citizens away from populated places to slow the spread of Covid-19, they may not have realized that they were also combatting other infectious diseases, such as the seasonal flu.

But they have been, according to data from Kinsa Health, a company that has sold or given away more than 1 million smart thermometers in the US. Kinsa collects anonymized thermometer readings (via its app, which users connect to the device) from its active user base to estimate the share of people that are ill in different geographies. By comparing current thermometer readings to historical trends, researchers have used Kinsa’s data to predict flu outbreaks weeks before the Centers for Disease Control and Prevention’s surveillance program, which uses hospitalization records.