The first version of the now-ubiquitous “flatten the curve” chart appeared in a 2007 report by the Centers for Disease Control and Prevention. The image, titled “Goals of Community Mitigation,” accompanied a list of radical recommendations to swiftly change American life in the event of a pandemic: close schools, cancel public gatherings, recommend voluntary isolation. The authors used a now-familiar term for the strategy: “social distancing.”

The colors have changed, but the chart then was more or less what we see today. Two ugly humps: one purple and steep, representing the outcome of a pandemic in which no interventions are taken; the other hatched and flatter than the first, depicting the outcome under a community-mitigation regime.

There were axes and legends, and Drew Harris, a professor of population health, would later add a line representing the capacity of the health care system, but in truth there was nothing particularly rigorous about the chart. It was a work of the imagination, too artless to be art but lacking the hard empiricism we expect of science. That in-betweenness is what made it so effective.

When public health crises emerge, the scientists tasked with helping people understand infectious threats need to overcome a huge perceptual obstacle: Pathogens can’t be seen by the naked eye. Since the advent of modern medicine, public health visualizations have been forced to find innovative ways to render invisible threats visible, and thus comprehensible.

In 1858, pioneering statistician and social reformer Florence Nightingale used a unique rose diagram to show that the primary cause of death for British soldiers in the Crimean War wasn’t fatal wounds on the battlefield but disease due to unsanitary living conditions. In 1854, when cholera broke out in London and no one knew how the illness spread, Dr. John Snow’s map of the locations of cases in the Soho neighborhood showed that the illnesses were centered around a single water pump on Broad Street.