Ragusa made everyone wait out the plague

When plague swept the Mediterranean in the mid-14th century, trading ports like Ragusa depended on ships and travelers arriving constantly from infected regions, and there was no way to look at an arriving traveler and know whether they carried the disease — the plague's incubation period meant a person could look and feel completely healthy while already becoming contagious. Inspecting people for visible symptoms at the point of entry, the obvious response, simply didn't catch the cases that mattered.

Turning away all trade and travel from affected regions entirely would have strangled the city's economy, since Ragusa's whole position depended on functioning as a working Mediterranean port. The city needed a way to keep trading with plague-affected regions without importing the plague itself, and no inspection method available at the time could reliably tell a healthy traveler from one who would turn out to be infected.

Held everyone from affected areas for 30 days, longer than the incubation period, so symptoms appeared before anyone reached the city.

Incubation period hides infection, so symptom checks miss contagious but healthy-looking travelers.

A 30-day wait exceeds typical incubation, so symptoms appear before entry.

Universal application avoids judgment errors and exceptions that would reintroduce risk.

When you can't detect a risk at entry, impose a holding period long enough for it to reveal itself.

The trentino spread to other Mediterranean port cities over the following decades, with the isolation period eventually extended to 40 days — the quarantino, the direct origin of the modern word 'quarantine' — and Ragusa followed up in 1390 by establishing a permanent Health Office to administer public health measures on an ongoing basis, one of the earliest institutions of its kind.