In part 3 of Guns, Germs, and Steel, Diamond traces the ways food production led to the development of the guns, germs, and steel that enabled Eurasians to conquer so many people around the world. One of the most important results of human domestication of farm animals was a transfer of diseases between animals and people. Smallpox, flu, plague, and many other diseases stem originally from infections in farm animals. This is significant because, according to Diamond:

The winners of past wars were not always the armies with the best generals and weapons, but were often merely those bearing the nastiest germs to transmit to their enemies.

Among germs, as among other forms of life, evolution selects those individuals that are most effective at surviving and producing offspring. In the case of microbes, this means that the individuals most effective at infecting new victims are most likely to survive. It makes little difference if the microbe kills the victim as long as the disease spreads. Many of the nastiest germs move through populations as epidemics. They spread quickly from person to person, make each person very sick for a short time, and leave victims immune for life—if they recover. The result is a highly infectious but largely immune population.

This kind of highly infectious disease cannot survive in most hunter-gatherer communities. Hunter-gatherer populations are too sparse. Germs that move through these populations too quickly tend to kill everyone and die out before they can spread further. Because of this, hunter-gatherer societies usually supported other kinds of germs but not highly infectious ones like smallpox and plague.

Germs were major contributors to European victories around the world, especially in the New World. From the time the Spanish arrived in the Americas, far more Native Americans died from disease than from battle. Importantly, disease killed many political leaders, which destabilized governments and left soldiers with low morale. Smallpox killed huge proportions of both the Aztecs and Incas. North America’s population was similarly decimated, in this case by overland arrival of disease that preceded the arrival of European colonists. Archaeologists now think that North America had a population of about twenty million before 1500 CE but lost at least nineteen million to Eurasian diseases.

More than a dozen major Eurasian diseases killed Native Americans, but there was no significant transfer of diseases from the Americas to Europeans. Many New World populations were large and dense enough to support infectious diseases, but these cultures had developed their population densities too recently for highly infectious diseases to arise. More importantly, the lack of domesticable animals in the New World left America’s population with fewer opportunities to pick up nasty germs from their stock.

Around the world, Eurasian diseases typically killed fifty to one hundred percent of the people European colonists contacted. In some areas, such as Africa, Asia, and New Guinea, explorers picked up new diseases in return. However, the conquerors’ diseases did more damage overall than the diseases of the people who were conquered.