Population density as a factor in crowd disease:

"Studies show that measles is likely to die out in any human population numbering fewer than half a million people. Only in larger populations can the disease shift from one local area to another, thereby persisting until enough babies have been born in the originally infected area that measles can return there."

"...True for other infectious diseases throughout the world. To sustain themselves, they need a human population that is sufficiently numerous, and sufficiently densely packed, that a numerous new crop of susceptible children is available for infection by the time the disease would otherwise be waning. Hence measles and similar diseases are also know as crowd diseases."

Obviously, crowd diseases could not sustain themselves in small bands of hunter-gatherers and slash-and-burn farmers. As tragic modern experience with Amazonian Indians and Pacific Islanders confirms, almost the entire tribelet may be wiped out by an epidemic brought by an outside visitor -- because no one in the tribelet had any antibodies against the microbe."

203

"The small population size of the tribelets explains not only why they can't sustain epidemics introduced from the outside, but also why they never could evolve epidemic diseases of their own to give back to visitors."

204

"In contrast, the crowd diseases...could have arisen only in the buildup of large, dense human populations. That buildup began with the rise of agriculture starting about 10,000 years ago and then accelerated with the rise of cities starting several thousand years ago.

205

"Agriculture sustains much higher human population densities than does hunting-gathering lifestyle -- on the average, 10 to 100 times higher. In addition, hunter-gatherers frequently shift camp and leave behind their own piles of feces with accumulated microbes and worm larvae. But farmers are sedentary and live amid their own sewage, thus providing microbes with a short path from one person's body into another's drinking water."

"Sedentary farmers become surrounded not only by their feces but also by disease transmitting rodents, attracted by the farmer's stored food. The forest clearings made by African farmers also provide ideal habitats for malaria-transmitting mosquitoes."

"Still more densely packed human populations festered under even worse sanitation conditions. Not until the beginning of the 20th century did Europe's urban populations finally become self-sustaining: before hen, constant immigration of healthy peasants from the countryside was necessay to make up for the constant deaths of city dwellers from crowd diseases."

Smallpox from Asia due to trade routes: "killed millions of Roman citizens between AD 165 and 180."

205

"The explosive increase in world travel by Americans, and in immigration to the United States, is turning us into a melting pot--this time, of microbes that we previously dismissed as just causing exotic diseases in far-off countries."

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Jared Diamond, Guns, Germs, and Steel: The Fates of Human Societies, (New York: W. W. Norton & Co.: 1997).