Yes, because they know if there is one that they've detected, it probably means there are others. And this individual travelled, and went to crowded events. As

Yes, because they know if there is one that they've detected, it probably means there are others. And this individual travelled, and went to crowded events. As of now they've found 4 additional cases. Thanks to the wonders of exponential spread you need to stop the spreading when the numbers are still small. Otherwise you fail miserably and kill more than half a million people.