Rain was falling on the night of Jan. 18, so the windows of the Tokyo party boat were shut. Inside were about 90 guests of a local taxi association who were celebrating the new year as the vessel floated down the Sumida River. Also on board, unbeknown to them, was a coronavirus capable of spreading ferociously.

It did just that. A driver in his 70s soon fell ill with fever; he later tested positive. The same day as his diagnosis, his mother-in-law died; she also was infected. Officials then discovered that 10 others from the boat were, too, including an employee who had served passengers from Wuhan, China. Still more who did not attend the party caught the virus after coming into contact with those who did.

As public health officials look for clues to one of the biggest uncertainties about the new coronavirus epidemic — whether it will eventually expand rapidly beyond its center in China — they are closely studying clusters of cases that have emerged recently in Japan.

[Update: Japan shocks parents by moving to close all schools in response to coronavirus.]

The issue has taken on more urgency as passengers have begun walking off a contaminated cruise ship in Yokohama where 634 people have tested positive for the virus and two later died. Experts fear that some who were cleared to leave an onboard quarantine could later test positive, spreading infections on land in the same way that the party boat has done.