I get that every time two sets of people have been isolated from one another, and then come back into contact, there is an opportunity for epidemics on either side. Why was the post-1492 epidemic exchange so massively one-sided? (I gather that 90% of indigenous Americans died, whereas a relative handful of Europeans died.)
Short answer:
"there is an opportunity for epidemics on either side."
Your question presumes that populations A and B each were equally endowed with diseases to which they were accustomed but which were lethal to the other-- but that's not the case. There was an asymmetry in diseases between American and Old World populations.
Discussion:
Old world populations had diseases which the New World population hadn't seen in 15,000 years. The New World populations in temperate latitudes didn't have much in the way of diseases novel for Europeans; there was an asymmetry of biological threat. Native North Americans had few diseases that could substantially harm Europeans-- Europeans brought with them several diseases devastating to indigenous people in North America.
So, for example, smallpox is devastating to indigenous American populations -- and there is no equivalent disease in the Americas which is devastating to Europeans. The only disease which _may_ have originated in the Americas and spread to Europe is syphilis, but that's nothing like the lethality of smallpox. If you look at deaths from disease in European populations-- both in the New World and the Old, you'll find that novel diseases from the Americas matter very little.
William McNeill's Plagues and Peoples remains a good introductory read on the impact of disease, and discusses this subject extensively. Its not without its faults -- recognized both at the time and expanded on subsequently-- but it remains a well written introduction to the subject by a first rate historian, albeit one who's treading unfamiliar ground.
You can observe that in Africa and India, indigenous people had lots of diseases that Europeans didn't, and Europeans often became sick and died of any number of diseases. This didn't happen in the Americas for the most part -- though in the subtropics and tropics, Europeans did encounter diseases like yellow fever and malaria. Though not unique to the Americas, these caused a great deal of disease and death among European arrivals to the Caribbean, for example.
Its notable that as one moves to the tropics, indigenous populations are a larger percentage of the population -- Guatemala is presently the American nation with highest proportion of indigenous population. This is due at least in part to the extent of disease suffered by Europeans in these warmer climates.
Sources:
McNeill, William. Plagues and Peoples, Doubleday 1976
Bhattacharya, Nandini. Contagion and Enclaves: Tropical Medicine in Colonial India. Liverpool University Press, 2012.
Brendan D. O'Fallon, Lars Fehren-Schmitz. "Native Americans experienced a strong population bottleneck coincident with European contact". Proceedings of the National Academy of Sciences Dec 2011, 201112563; DOI: 10.1073/pnas.1112563108
Crosby, Alfred. The Columbian Exchange. Greenwood:1972
McNeill, J. R. “Yellow Jack and Geopolitics: Environment, Epidemics, and the Struggles for Empire in the American Tropics, 1640-1830.” Review (Fernand Braudel Center), vol. 27, no. 4, 2004, pp. 343–364.
Engelen, Theo, et al., editors. Death at the Opposite Ends of the Eurasian Continent: Mortality Trends in Taiwan and the Netherlands 1850-1945. Amsterdam University Press, 2011.
The Burdens of Disease: Epidemics and Human Response in Western History, by J. N. Hays, REV - Revised ed., Rutgers University Press, New Brunswick, New Jersey; London, 2009
I answered a very similar question a week ago. I hope the answer helps. It boils down to plagues not existing in pre-Columbian America due to the lack of cattle