"13 of the 41 cases had no link to the marketplace" How do they know, when its know that the virus can be spread before symptoms kick in? (https://edition.cnn.

"13 of the 41 cases had no link to the marketplace" How do they know, when its know that the virus can be spread before symptoms kick in? (https://edition.cnn.com/2020/01/31/health/coronavirus-asymptomatic-spread-study/index.html https://edition.cnn.com/2020/01/31/health/coronavirus-asympt...)