Why did Venice require ships to wait 40 days before entering port during plague outbreaks?

In the 14th century, Venice implemented a mandatory isolation period for ships arriving from the Levant to prevent the spread of bubonic plague, which was frequently introduced via maritime trade.1

The practice, known as quarantine, was designed to isolate ships and their cargo for a sufficient duration to allow any potential infection to manifest and dissipate. While the initial requirement was 30 days—referred to as trentina—this was eventually extended to 40 days, or quarantina.1

The choice of 40 days is historically attributed to the symbolic period that Jesus Christ and Moses spent in isolation in the desert. This Venetian system became the foundational model for disease control across Europe, leading to the establishment of specialized isolation stations known as lazarettos.1