What an Epidemic Does to a Society

An epidemic is usually remembered by how many it killed. For a historian trying to work out what it did, the more useful questions are who it killed, which institutions it emptied, and what it did to the price of labour and land.

Start with the distinction that organises everything else. In an endemic disease, most adults survived exposure in childhood, so an outbreak takes the young and the immunologically unlucky. In a virgin-soil epidemic, a disease entering a population with no prior exposure, mortality falls across every age group at once. That is why such events are socially catastrophic out of all proportion to their death tolls: they remove, in a single season, the people who know how to plant, negotiate, heal and govern, along with everybody else.

The Columbian case is the extreme one and is usually told as a single event, which it was not. It was a sequence (smallpox, then measles, then typhus, then influenza) arriving over decades into societies already destabilised by conquest, displacement and the disruption of planting. That compounding is why the cumulative losses are so much larger than any single outbreak can explain, and why estimates vary so widely: what is being measured is a process rather than an occurrence.

Age structure decides the economics. A disease that takes the elderly removes institutional memory and frees property, and can leave survivors materially better off. One that takes children removes the next generation of workers and shows up in the record twenty years later. One that takes working adults does the most immediate damage per death, which is what makes the 1918 influenza so unusual: its mortality curve had a pronounced peak among people in their twenties and thirties, probably through an immune overreaction, and it therefore struck the most productive group and the parents of young children at once.

The measurement problem is worth stating plainly because it recurs. Case counts measure testing at least as much as they measure disease, so a place that tests little looks healthy. Recorded causes of death depend on who was diagnosing and what they were permitted to write. Both are political. The instrument historians reach for instead is excess mortality: deaths above the expected baseline, whatever the stated cause. It is cruder, it is slower, and it does not require anybody to have identified the disease correctly, which is precisely the point. The same technique works on seventeenth-century parish burial registers and on modern national statistics, which is unusual and makes very long comparisons possible.

What follows an epidemic is rarely a return to the previous arrangement. Labour becomes scarce or land becomes abundant; inheritance jumps a generation; institutions that depended on particular people stop functioning; religious and political authority is either strengthened by having an explanation or discredited by not having one. The disease arrives into a society and leaves a different society behind it, and the interesting history is nearly all in that second sentence.