The issue has taken on greater urgency as hundreds of passengers disembark from the Diamond Princess, the cruise ship in Yokohama where 621 people have tested positive for the virus. Japan declared the ship’s two-week quarantine over, despite an uptick in cases among the passengers still onboard.

Alarmed officials are rushing to learn more about how the virus is transmitted, including how many of those infected experience mild symptoms or none at all, and to what extent it can be spread by people who experience no symptoms.

Virologists see two likely explanations for the spread of clusters. In one, a “superspreading event,” a person who has the propensity to spew more germs than others transmits the virus to a large group of people.

Alternatively, people can independently catch a virus from contaminated surfaces. It is unclear how long the new coronavirus can survive on surfaces, but studies of other such viruses have found they can stay active for a week or more.

On Thursday, a Japanese health ministry official said two infected passengers who were quarantined on the ship had died. The two, both Japanese, were an 87-year-old man and an 84-year-old woman, the Japanese broadcaster NHK reported. Both had underlying health issues, the broadcaster said.

The authorities have said they are releasing only people who have tested negative for the virus — though testing has been unreliable — and are showing no symptoms. But experts on infectious diseases have pointed to deficiencies in the quarantine protocols on the ship and questioned the decision to let them go free.