After idling for days off the California coastline, the Grand Princess cruise ship docked in Oakland on Monday — along with its roughly 3,500 passengers, 21 of whom have tested positive for COVID-19. Passengers will be tested and quarantined at U.S. military bases or back in their home countries.

At a moment of great global fear — particularly surrounding cruise ships, given the more than 700 coronavirus cases and eight deaths from an outbreak on the Diamond Princess cruise ship, docked off Japan — some have expressed reluctance to let potentially infected passengers disembark. Even President Trump said last week that his personal preference would have been to keep them on the Grand Princess.

Epidemiologists, however, say that keeping sick people on a ship in an attempt to isolate disease can “amplify infections.”

“Quarantine vessels went out of business a long time ago,” as University of California, Berkeley epidemiologist Arthur Reingold recently put it to TIME. 

That fact shows how far the science on effective quarantine science measures has come. In fact, the term quarantine has maritime origins, coming from quaranta giorni, Italian for “forty days,” the length of time ships from infected ports had to anchor in Venice during the 14th century bubonic plague epidemic, according to the CDC.