Stopping the spread of a rapidly emerging disease takes masterful medical detective work, including tracing the people who have been infected and figuring out their web of contacts, steps that are vital to understanding how it’s being transmitted. US public health officials are following those trails to quickly detect new cases of the Wuhan coronavirus, including the report on Thursday of a sixth US infection—the husband of a woman who became ill after traveling from China back home to Chicago—which was followed by a seventh, in California, on Friday.