Wave over wave: How water imagery has helped us understand (and predict) disease

This column is an instalment in our series Apocalypse Then, in which cultural historian Ainsley Hawthorn examines the issues of COVID-19 through the lens of the past.

Since the beginning of the COVID-19 pandemic we've been hearing from public health officials about waves of disease: which wave we're currently in and how many more we can expect.

Despite the widespread use of the term, though, pandemic "waves" are a 19th-century metaphor with no firm scientific definition.

Wave imagery was first used to understand the behaviour of disease in the middle of the nineteenth century, when British imperial governments began collecting and charting disease data.

Reviewing the incidence of fevers in the Americas in the early 1800s, Robert Lawson, deputy inspector-general of hospitals for the British military, wrote: "There seem to be a series of waves, generated in southern latitudes, which flow to the north or north-westward in succession, leading to an increase of fever at every point over which they pass; and, in some instances, these are so close, that Canada is under the influence of one, the West Indies of the following one, and a third is apparent at the Cape."

He called this pattern the "Pandemic Wave."

As a metaphor, waves had the capacity to capture both the movement of infectious diseases in space – travelling from one place to another like a wave sweeping up over a beach – and their prevalence over time – case counts that rise and fall in a population like waves on the ocean.

In 1899, when a new disease swept across the world, the press used this concept of pandemic waves to explain it. The Grippe or Russian flu, as the illness was known at the time, may, like COVID, have been caused by a novel coronavirus, and it progressed in at least three surges, each about a year apart.

By the time the 1918 influenza pandemic came along, waves had transitioned from a means of describing a pandemic's behaviour to a method for predicting it.

Health officials not only acknowledged current waves but speculated on the likelihood that the disease would return in future ones.

The 1889 and 1918 pandemics have informed how scientists model pandemic diseases to this day. Those outbreaks, though, with three waves each, never hinted that Canada would find itself, after two years, in its sixth wave of COVID-19.

How common is it, then, to experience so many waves of disease?

The number of waves in a pandemic can vary widely. Some infectious illnesses, like HIV/AIDS, don't spread in a wave pattern at all; other viruses that have the potential to cause waves sometimes expand in one steady swell instead, as influenza did in the pandemic of 1968.

At the far end of the spectrum there are prolonged pandemics that persist through surge after surge.