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	May 11th, 2009 - 05:51 am

The argument that American populations were particularly vulnerable is based on a combination of the low founding population, low HLA diversity (their immune systems varied far less from person to person), and the long period of isolation (also applicable to many/most island populations). Not to mention contemporary primary sources that point out excessive death rates compared to Europeans. Selection only works if there are existing beneficial alleles in the population to select for.

(Here's a table estimating the diversity on several immune system loci by population, by the way: http://www.genetics.org/cgi/content/full/156/4/2119/F2)

And general scientific consensus is that the suite of Eurasian diseases were -not- extant on the American continents. There's no real argument about this at all, actually. Smallpox only showed up period around 12,000 years ago, and wasn't in Europe in ancient Greek times. Many of the diseases in question post-dated the period of isolation, only could become endemic in urban environments, and/or were acquired from livestock. I'm pretty sure measles was introduced to Australia by colonists too.

European populations had to deal with diseases spreading out of the Middle East and Africa all the time. Since new diseases were related to old ones, adaptations to previous disease waves were semi-helpful in responding to new ones. Even non-related diseases could do this. There's a good argument that the European HIV-resistance gene, CCR5, arose due to the Black Death, for instance. Because the Americans were relatively disease-free, they didn't have the constant selection pressure of the Old World, so even useful adaptations could be lost due to drift.

And even "resistant" European populations had massive mortality due to smallpox.

Yes, a population (and particularly an isolated one) can be killed off due to disease. It happens all the time in other species. Look what the chytrid fungus is doing to frogs.