Much of the research involving the coronavirus transmission has concentrated on contact between members of the same households, medical professionals and clusters within communal living facilities, such as hospitals, nursing homes and jails. But the Chicago case study, highlighted in the Centers for Disease Control and Prevention’s weekly mortality report, explains how gatherings — large or small — can lead to a proliferation of coronavirus cases between households in a community. The patient identified as the original carrier in the cluster didn’t live with any of the patients he’s believed to have spread the contagion to.