Jails and prisons are major sites of novel coronavirus (SARS-CoV-2) infection. Many jurisdictions in the United States have therefore accelerated the release of low-risk offenders. Early release, however, does not address how arrest and pretrial detention practices may be contributing to disease spread. Using data from Cook County Jail-one of the largest known nodes of SARS-CoV-2 spread in the United States-in Chicago, Illinois, we analyzed the relationship between jailing practices and community infections at the ZIP code level. We found that jail-community cycling was a significant predictor of cases of coronavirus disease 2019 (COVID-19), accounting for 55 percent of the variance in case rates across ZIP codes in Chicago and 37 percent of the variance in all of Illinois. Jail-community cycling far exceeds race, poverty, public transit use, and population density as a predictor of variance. The data suggest that cycling people through Cook County Jail alone is associated with 15.7 percent of all documented COVID-19 cases in Illinois and 15.9 percent of all documented cases in Chicago as of April 19, 2020. Our findings support arguments for reduced reliance on incarceration and for related justice reforms both as emergency measures during the present pandemic and as sustained structural changes vital for future pandemic preparedness and public health.