On ships at sea, respiratory viruses such as influenza and enteric pathogens such as norovirus can spread quickly.3,4 In the early weeks of the pandemic, several outbreaks of Covid-19 occurred on cruise ships, most notably on the Diamond Princess.5,6 The medical department of a ship can be overwhelmed quickly by a major outbreak of disease, as is similarly seen with health care facilities in civilian communities.7 The shipboard environment on naval vessels is generally more confined. Typically, enlisted crew members sleep in open bays packed with dozens of tightly spaced bunks, work in densely populated areas, and congregate in gathering points such as the gyms and galleys (Figs. S1 and S2 in the Supplementary Appendix, available with the full text of this article at NEJM.org). These conditions probably facilitated the transmission of SARS-CoV-2, as evidenced by the higher likelihood of Covid-19 among enlisted crew members than among officers (Table 1).