Quarantine institutions

Why did cities that knew nothing about germs invent a waiting period that actually worked?

In 1377 the city of Ragusa — Dubrovnik — ordered that arrivals from plague districts be held thirty days on the island of Mrkan or at Cavtat before entering the town. Venice later stretched the period to forty and built a dedicated isolation island for it, and the Italian word for forty, quaranta, gave us the name we still use.

None of these officials had any idea that a bacterium existed. The prevailing explanations were corrupt air and some vague notion of contagion by touch or gaze. And yet the rule they wrote does something a modern epidemiologist would recognise as correct. That is the puzzle worth sitting with. How does a wrong theory produce a right procedure?

Let us not answer by admiring their intuition. Let us ask what the procedure actually requires you to know.

To design a waiting period, you need one fact: that an apparently healthy person from an infected place may become dangerous after some delay, and that the delay has an upper bound. Nothing in that sentence mentions a cause. You can observe it purely from the outside — ship arrives, crew looks well, crew falls ill nine days later, town falls ill after that. The mechanism is invisible; the interval is not.

Now notice how forgiving the design problem is. Bubonic plague incubates roughly two to eight days; the pneumonic form is faster still. A rule of thirty or forty days does not need to be tuned. It is four or five times the quantity it must exceed. Ask yourself what that means for a city guessing: a guess that is generously long is indistinguishable in effect from a correctly calculated one. The policy is robust to enormous error in the underlying theory, because the only parameter that matters was set with a wide margin.

Where did the margin come from, if not from measurement? Largely from elsewhere. Forty had ready-made authority — the forty days of Lent, the Hippocratic tradition of critical days, the general medieval fondness for the number. So the specific figure was borrowed from religion and inherited medicine, not derived. Which is the point: the institution did not need to be derived to be right.

Then the harder question. If the number was arbitrary, why did the institution spread rather than dying with the fashion that produced it?

Here the reasoning move is adaptation, and it is worth being precise about it rather than waving at "evolution". Adaptation needs three things: variation, a difference in outcome, and retention. All three are present. Variation: Mediterranean port cities tried many things — expulsions, processions, bills of health, cordons, waiting periods of differing length, in different combinations. Difference in outcome: a city whose harbour rules failed lost a fifth of its population and its trade with it, and everybody in the region watched. Retention: these were written institutions — statutes, health boards, permanent buildings — copied deliberately by neighbouring republics. Venice's health magistracy became a standing office; the lazaretto became a piece of architecture other ports reproduced.

So the selection is not on beliefs at all. It runs on the observable consequence of the rule, and it can therefore drag a correct practice along behind an incorrect explanation indefinitely. The explanation was free to stay wrong because nothing was testing it. The rule was not free to stay wrong, because ships kept arriving.

Two honest qualifications. First, the practice was never clean: quarantine leaked constantly, was enforced against the poor and the foreign more than the rich, and repeatedly failed outright — Marseille in 1720, western Europe's last great plague, came in on a ship whose quarantine was cut short for commercial reasons. Second, historians still argue about how much credit quarantine deserves for plague's retreat from western Europe after that date, against changes in rodent ecology, housing and trade routes. The mechanism above explains how the institution could work; it does not by itself settle how much of the epidemiological record it explains.

The temptation is to conclude that understanding is optional — that trial and error will find the right practice anyway. It will, but only under conditions that were unusually favourable here. The feedback was fast enough to attribute, the outcome was catastrophic enough to notice, the practice was cheap enough to keep, and the target parameter was crude enough that a superstitious guess overshot it.

Change any of those and the ratchet stops. A disease with a six-month latency would defeat a forty-day rule and no city would ever learn why. A practice whose benefit arrived a generation later would not be attributed to it. And — the sharpest limit — because nobody understood the mechanism, nobody could transfer it. The same authorities who quarantined ships correctly also burned houses, killed dogs and expelled Jewish communities with the same confidence, since their theory could not distinguish the measure that worked from the ones that did not. Adaptation gets you a working rule. Only the mechanism tells you which of your rules is the working one.

• How bills of health and passports of clean provenance turned quarantine into an information system rather than only a barrier.
• Why the 1851 International Sanitary Conferences struggled for decades to agree rules, and what changed once the pathogen was identified.
• Which other pre-scientific institutions show the same shape — a correct parameter carried by a wrong theory.

Every term the collection defines is gathered in the glossary.