Why was the Columbian exchange so lopsided: potatoes one way, smallpox the other?

The Columbian exchange, the swap of plants, animals and germs between the Americas and Afro-Eurasia after 1492, was lopsided because the two worlds had very different histories. Eurasia had domesticated far more animals and had lived for much longer in dense cities, so it had bred many more infectious diseases. The peoples of the Americas had never met those diseases and had little or no immunity. Crops flowed generously in both directions, but germs flowed mostly one way, and Old World diseases cut the Indigenous population of the Americas by 80 to 95 percent.

The historian Alfred Crosby gave the exchange its name in 1972. On the dinner table, its gifts were enormous. Potatoes, maize, tomatoes, cassava and chili peppers left the Americas and spread around the world; wheat, rice, sugar cane and livestock went the other way. Before 1500 nobody grew potatoes outside South America, yet they later contributed an estimated 12 to 25% of the population growth of Afro-Eurasia between 1700 and 1900.

The cost in the Americas was catastrophic. Hispaniola had probably at least 500,000 people before contact and fewer than 500 by 1526, from disease combined with Spanish exploitation. Crosby argued that historians missed this for a long time: they credited Cortés's genius as a soldier, not the smallpox that travelled with his army.