GGS - Chapter 11

The next section of the book will explain how the ultimate cause of societal growth (food production) led to the proximate causes of societal triumph (germs, guns, steel, technology, writing, government, etc.). First up on the list is the advent of "crowd diseases". As opposed to the diseases present in hunter-gatherer societies that infected a same, steady amount of people, crowd diseases occur in epidemic waves. This is due to four main attributes: they spread quickly from person to person, are acute (either death or get well quickly), immunity is acquired after infection (so that that disease has to wait until a fresh generation matures to spread a new epidemic) and that they are confined to humans.

The sedentary farming lifestyle (and later dense cities) helped foster these crowd diseases. The localization of feces and its use in agriculture helped, as well as trade routes and fostered more spread. Eventually, susceptible would die and those carrying genetic resistance would allow that trait to spread in the population. But how did these acute crowd diseases evolve from the hunter-gatherer diseases? The domestication of social animals brought the crowd diseases of animals to humans.

This leads to an important question: why did Eurasian germs cripple North and South America and not the other way around? Minor reasons could be the earlier start of dense, sedentary communities in Eurasia and the significant trading that occurred to allow the spread of disease. However, the main reason was that the Americans had little crowd disease starting material. The paucity of domesticated social animals in the Americas led to few candidate microbes that could evolve to infect humans. Thus, even though the Aztec and Inca civilizations (conquered by Cortez and Pizarro, respectively) were relatively dense, the lack of domesticated animals prevented the cultivation of crowd diseases.

An interesting note in this chapter was the explanation of symptoms. Specifically, symptoms are usually just mechanism to spread disease (diarrhea, genital worts, coughing for airborn diseases and rabid biting behavior for rabies).