In the mid-18th century, ships were routinely making long voyages out to sea, where a disease, known as scurvy, was killing British sailors faster than any enemy fleet was. While the symptoms were known — fatigue, sore and bleeding gums, loosening teeth, and joint pain — its cause was not.

In the first part of Exploring Epidemiology, we met early thinkers who began connecting observation to disease. In the second part, we saw how the Black Death forced cities to build the first public health infrastructure by counting the dead, isolating the sick and establishing quarantine. Counting the dead was one thing, but what did those numbers mean? It took a curious tradesman to find out.

In the mid-14th century, the Black Death was sweeping across Europe. Towns emptied, mass graves replaced cemeteries, and within just a few years, an estimated one-third of the continent's population was gone. It remains one of the most devastating epidemics in human history; for those who lived through it, nothing would ever look the same. And yet, out of that terror and uncertainty came some of the most important public health innovations ever developed — some still in practice today.