How the ‘Father of Epidemiology’ Made the Connection Between Disease and Geography

On the first day of his epidemiology course at the London School of Hygiene & Tropical Medicine, Professor Paul Fine tells the story of John Snow. Snow was a doctor in Victorian London, and he was a man, people say, as pure as his name—a vegetarian, a teetotaler, and a bachelor. When Queen Victoria went into labor with her eighth child, Prince Leopold, John Snow was by her side, delivering anesthesia. But Snow led a double life. Far from Buckingham Palace, he trawled the streets and slums of Victorian London on an extracurricular mission, trying to figure out how cholera was spreading.

Cholera probably originated in India, before spreading through the Middle East and Russia, but it only arrived in England in 1831. At the time, there was no real understanding that germs, or microorganisms, spread disease. Instead, the “miasma theory”—the belief that disease came from vapors, or smells, arising from decay—dominated among medical experts. Smells, in other words, weren’t just signs of disease; they were the disease itself.

Snow, who had treated coal miners struck with the disease as a doctor’s apprentice in Newcastle, knew that cholera symptoms started in the stomach, not the nose. He hypothesized correctly that the disease was actually spread through drinking polluted water and eating with unclean hands.

Snow’s evidence was circumstantial, but he knew something else about his neighbors struck down by cholera, something he didn’t learn in medical school. As Steven Johnson points out in his engrossing book, The Ghost Map, Snow was not just a public health tourist, “goggling at all the despair and death, and then retreating back to the safety of Westminster or Kensington.” He lived only a few streets away from Broad Street, the center of the epidemic. And while Snow was now a doctor who attended the queen, he had grown up poor. So unlike many doctors from more privileged families, he didn’t blame disease on the bad habits of the lowest classes. “The poor were dying in disproportionate numbers not because they suffered from moral failings,” Johnson writes. “They were dying because they were being poisoned.”