Frankly put, the steady diffusion of disease strengthens large populations by gradually eliminating humans with weak immune systems and replacing them with humans with stronger immune systems. While the spread of disease has caused countless deaths, it also acts as a net benefit to dense, agriculture-based societies. One “side effect” of trade between societies-but also a form of diffusion in its own right-is the spread of disease, since societies exchange germs and viruses when they interact. This is one reason why China, a highly technologically advanced society that deliberately cut itself off from other societies during the 15th century, fell behind Europe, a less technologically advanced region that nonetheless benefited from diffusion between itself many other cultures. No matter how advanced and self-sufficient a society might be, there are big historical advantages to trading with neighbors. In many ways, Diamond suggests that a society would be better off in the long run with normal citizens and connections to many neighbors than with brilliantly talented citizens but no neighbors to trade with. A society that trades with its neighbors can remain on the cutting edge: either the society itself discovers an important resource or idea, or a neighboring society does and the first society acquires it through trade. Trade is particularly important because it often involves the literal exchange of goods and ideas, meaning that a society that trades with its neighbors can benefit from its neighbors’ natural resources and its intellectual breakthroughs.

One of the most important and representative forms of diffusion that Diamond discusses is trade. One of the most important aspects of Diamond’s theory of geographic determinism is the concept of diffusion: the many different ways that technologies, ideas, goods, and resources are transported and spread both within a society and between separate societies.

Barriers to diffusion of disease