This article was published on September 18, 2021, at NEJM.org.

Photo credits:

St. Michael’s Hospital Archives, Toronto (operating theater). Raymond Coyne, courtesy of Mill Valley Public Library (group of mask wearers).

F. Ray Moulton, Aerobiology (droplet and aerosol transmission); reprinted with permission from AAAS. John Anderson (mask protests).

Image sources and recommended readings are listed in the Supplementary Appendix, available at NEJM.org.