When a crew member fell seriously ill, the vessel returned to port, and almost everyone was tested for the virus again. The before-and-after results for 120 of the crew members were made available to Bloom and colleagues, who published a study about them in The Journal of Clinical Microbiology in August. In addition to the P.C.R. tests, the pre-voyage screenings also looked for neutralizing antibodies, or proteins generated by the immune system after exposure to the virus, which suggest that a person has been infected previously. Three crew members, it turned out, had those antibodies at the start of the trip. Of the 117 crew members who did not, 103 tested positive for the virus when they got back to shore — an 88 percent infection rate. If you were to randomly select three names from the ship’s manifest, the odds that all three would have tested negative are about 0.2 percent. Yet all three sailors with antibodies were spared.