Quarantine is forty days long

The word carries its own duration. A fourteen-day quarantine is, etymologically, a contradiction.

What everyone sees

A generic word for isolation with no fixed length attached.

What is actually happening

During the plague years, Venetian-controlled ports held arriving ships offshore before letting anyone land. The practice began at thirty days — trentino — and was extended to forty, quaranta giorni, giving quarantena and eventually quarantine. Forty was chosen partly on medical grounds and partly for its heavy religious symbolism: forty days of Lent, forty days of the flood, forty days in the wilderness.

Why it stays hidden

Because the word outlived its number. Modern quarantine periods track incubation times, not Venetian scheduling, so the forty sits inside the word doing nothing but waiting to be noticed.