As severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections (causing coronavirus disease 2019 [Covid-19]) spread globally, uncertainty surrounds estimates of the true number of infected persons, which is crucial to determining the severity of infection and the incidence of mild or asymptomatic cases and their possible transmission. Modeling estimates suggest that in Wuhan, China, the city with the most Covid-19 cases, there are substantially more cases than were officially reported, because milder cases may not have been captured in hospital-based surveillance. Data on travelers returning from areas with cases of Covid-19 could be useful in estimating its incidence.