The worst place to have an epidemic, like a fire, is in close quarters far from help, such as a ship on the high seas. Unless the vessel is a very large one, no doctor, nurse, or proper medicines will be available, nor is it likely that they will be available for miles around. Crew and passengers will often have a choice between a cutting wind or air warm and germ-laden from passage through a dozen pairs of lungs. The proximity of all crew members to one another will make it quite likely that a high proportion will come down with any virulent breath-borne disease all at once, leaving the operation of the vessel in the hands of people tormented by high fevers and suffering from extreme prostration. Flu brought several ships, at least, very close to the brink of castastrophe.

The schooner Leverna, under Captain Robert Wharton, sailed out of Gloucester, Massachusetts, in mid-September 1918 in search of halibut—and sailed right back a few days later with her entire crew of 22 sick with flu. The first cases received by the United States Naval Hospital Number 13 in the Azores in September were off the Japanese steamer Shensi Maru, adrift in mid-Atlantic with a large number sick, several dead, and no doctor. On October 17 an unnamed vessel arrived at Le Havre, France, with 74 of a total crew of 78 sick with flu.

The record of Spanish influenza afloat is clearest and most complete for navies. It hit the United States Navy hard; perhaps as high as 40 percent of its personnel had influenza in 1918.