One way to explain the complexity and unpredictability of historical systems, despite their ultimate determinacy, is to note that long chains of causation may separate final effects from ultimate causes lying outside the domain of that field of science.
Jared DiamondMeasles and TB evolved from diseases of our cattle, influenza from a disease of pigs, and smallpox possibly from a disease of camels. The Americas had very few native domesticated animal species from which humans could acquire such diseases.
Jared DiamondEurasia ended up with the most domesticated animal species in part because it's the world's largest land mass and offered the most wild species to begin with.
Jared Diamond