The first recognizably modern study of epidemics was done in the mid-1800's by Jon Snow, a physician. He was living in London, England at the time of what became known as the great Cholera pandemics. I want to talk about, about his story, because it shows how patterns based on hard data can be used to infer epidemic processes. Moreover, Snow's cholera stories show what you can do, if you think about disease dynamics and transmission, instead on focusing just on the microbe or on the patient. Records of cholera-like deaths go back 25 centuries. The disease begins with severe vomiting and diarrhea which rapidly causes serious dehydration. People can die within a few days, or even a few hours, of the first symptoms. Today, with proper medical care, fewer than one percent of people with cholera die, but without medical care, more than half die. The first cholera pandemic in Europe happened around 1830, probably seeded from India. The second pandemic happened around ten years later, and ravaged Jon Snow's part of London, with thousands dying. At the time, no one knew what caused the disease. It's hard to imagine today that people knew nothing of germs. But the so called germ theory of disease was decades away. The most common theory during those pandemics was that cholera was caused by foul air. Stench from decaying animal and vegetable matter. They also thought that Cholera was a disease of blood because people with Cholera had very thick blood. In 1849, Jon Snow published his theory that Cholera must be a disease of the digestive system because it caused vomiting and diarrhea. The thick blood was a side effect, he said, due to dehydration. There's no reason that something must therefore be coming in the mouth, and out with the feces. We now know he was complete right. Cholera is caused by a gut infection with a bacteria called Vibrio cholerae. It is shared in feces so it is passed bum to mouth as it were via dirty water. But, when Snow proposed all this, he was largely dismissed. His chance to test his ideas came when cholera returned to London in the next pandemic. In what Snow described as an experiment on the grandest scale, he looked at where people who died of cholera in South London got their water. He discovered that people were fourteen times more likely to die of cholera if they got their drinking water from the Southwark and Vauxhall water company than if they got it from the Lamberth water company. Now both companies got their water from the river Thames, which must've been a disgusting cesspit in those days. But the Lamberth water came from further upstream, above the places where most of the human excrement entered the river. To Snow, this showed that cholera was waterborne, and that sanitation was the key to preventing it. Snow also did another quantitative study around the same time, and it is this that he is most famous for. Near his house in Soho, more than 500 people suddenly died of cholera. Snow trudged the streets around golden square neighborhood, asking where people got their water, and whether they had cholera or not. This is the resulting map, one of the most famous in epidemiology. Snow marked every death on the map. There were several pumps in the area, but almost all the deaths surrounded the Broad Street water pump. Snow hypothesized that the pump was the source of the contaminated water. He was able to nail his hypothesis by looking at the apparent contradictions. People working across from the pump in the Lion Brewery had not got sick. Snow discovered that they seldom drank the pump water; they preferred to drink the beer they had made. People who had died far from the pump turned out to have passed it on the way to work or to school. The final kicker was the widow Suzetta Eli who lived and died miles from the pump. It turned out she so loved the colder, carbonated water that came from that pump, she used to get it delivered. It killed her and a niece who was visiting her at the time. With these data in hands, Snow demanded the handle of the pump be removed. It duly was, the pandemic subsided, and Snow went into the history books as the father of modern epidemiology. There are two additional bits of the story I really like. The first is a discovery of how the well got contaminated with cholera bacteria in the first place. One Reverend Henry Whitehead was convinced that Snow was wrong, and that cholera was caused not by dirty water, but instead by God's divine intervention. In trying to place the blame on God, Reverend Whitehead discovered patient zero for the epidemic. Five year old Francis Lewis of 40 Broad Street died of cholera just before the epidemic started. Her mother washed the vomit and diarrhea into the cesspit right next to the pump. It's not clear how Francis herself got cholera, but everyone else got it from the water she contaminated. So if God was responsible for what Snow called the most terrible outbreak of Cholera which ever occurred in this kingdom, He did it via Francis. The second edition of the story, which I really like, is that Snow himself was unsure just how much difference removing the pump handle had made. He knew that cases were starting to come down anyway, partly because many people had died, partly because many others had left the area for fear of dying. Today we know that preventing people from drinking contaminated water works. Snow was right to be cautious about his data. To understand the impact of any disease control measure, you need to understand the disease dynamics. You need to ask what would have happened anyway. Snow found his data on the Southwark, Vauxhall water company much more persuasive than the hand removal experiment. And so do I. The bottom line of all of this, by thinking about the transmission, by gathering data on the cholera deaths in space and time and with some stunning reasoning, Snow was able to correctly deduce where in the body the disease was caused, what that meant for likely transmission routes, and what that meant for disease risk. From all of that, John Snow was able to save lives, with no idea of what a bacterium was. Without treating a single patient. This is the power of thinking about disease dynamics, and the need for hard data. What Snow did was the first analysis of, today, what we would call big data. Let me add a postscript. Cholera continues to provide important lessons in epidemiology, even today. After the earthquake in Haiti in 2010, there was a Cholera outbreak. The cases were mapped, just like John Snow had done. The first cases were down stream from a United Nations base. Third generation real-time DNA sequencing showed that the bacterial strains responsible for the Cholera outbreak were closely related to a strain circling in Nepal, from where some of the UN troops had come. I'm guessing John Snow would not be surprised to see cholera pop up when sanitation collapses. Or to learn that diseases don't respect borders. But he surely would be staggered that we can trace epidemics across the world without the shoe leather epidemiology that he pioneered.
