The Ghost Map: The Story of London's Most Terrifying Epidemic--and How It Changed Science, Cities, and the Modern World - Steven Johnson Cholera is a nasty little bug. Once ingested, it forms colonies on the intestinal wall, begins to reproduce with ferocious speed, and proceeds to trick the cells into excreting water rather than absorb it. It doesn't really matter of the host dies soon, because millions of new little cholera bacteria rush out of the host with the excreta waiting for the next person to ingest some excrement. That is the key. The only was to get cholera is by ingesting the excrement of another person so infected. Now you might say, whoa, that's more than I really wanted to know and I have no intention of so doing anyway. Well, you're right, all homo sapiens have a predisposition NOT to do just that, but given the rise of cities, the closeness with which we live, the relative ease of transportation, and the total misunderstanding of basic sanitation that existed until the 20th century, it was inevitable that the little buggers would escape their original habitat along the Ganges River.

Johnson discusses the interrelationship of the rise of cities, alcohol tolerance as a genetic adaptation to increased agriculturalization. Drinking water could be quite hazardous, but drinking beer and other alcoholic drinks had survival value from a natural selection standpoint because the fermentation process and alcohol killed off many harmful bacteria. Since alcohol is a poison and ill-tolerated by many, the speculation is that as agriculture and cities began to predominate, those who could tolerate alcohol better than others survived to reproduction age.

In another of those little actions that are intended to benefit, but which have unintended consequences, the change in use of sewers in London, inadvertently laid the groundwork (pun intended) for the cholera epidemic. Sewers had been designed to channel away rain water to and help prevent flooding in the city. In fact, it was prohibited to dump anything in the sewers and the Thames had been teeming with fish and quite clean. As the population increased, waste accumulated, and the aroma of piles of excrement in basements and elsewhere gave the miasmatists (those who believed disease was transmitted in the air) food for thought (the puns just keep rolling along.) So they had the brilliant idea of using the sewer system to wash the excrement out of the city and into the river which soon became foul. As it had also been the source of drinking water, the transmission of the cholera bacteria was efficient and inevitable.

Snow's rational approach to discovering the cause of the disease is remarkable in other ways. It had been common (a mythos that still is often heard today) to blame disease on lack of moral fiber. Since most of the victims were poor, and we all know that the poor are morally unfit, the victims themselves were somehow responsible for the illness. Snow rejected that possibility, rationally looking at evidence and building his case for the water-bourne nature of the disease. Johnson turns a nice metaphor in describing Snow's discovery: " great breakthroughs usually happen in practice. It is rarely the isolated genius having a eureka moment alone in the lab. Nor is it merely a question of building on precedent, of standing on the shoulders of giants, in Newton's famous phrase.Great breakthroughs are closer to what happens in a flood plain, a dozen separate tributaries converge and the rising waters lift the genius high enough that he or she can see around the conceptual constructions of the age."

How the source of cholera epidemics in London in 1854 was identified and explained is the subject of this engrossing work. A very good companion book to read with this one is Yellow Fever, Black Goddess: The Coevolution of People and Plagues which has an excellent section on modern cholera treatments.