As the coronavirus continues its global march, it’s clear that a significant number of cases are very mild or cause no symptoms at all. Yet people with these stealth infections can still shed the virus and spread the disease, creating a swirl of confusion about how many cases may actually exist worldwide—and how we can hope to contain the pandemic.

Early estimates for asymptomatic cases are inconsistent. On Sunday, Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, suggested that the number could be as high as 50 percent, double the tally previously estimated by the Centers for Disease Control and Prevention. Other studies have reported estimates as low as 18 percent aboard