Quote Originally Posted by Klearchos View Post
The most notable example is the colonization of the Americas, where both Native Americans and Europeans suffered greatly from such diseases.
In respect to diseases and immunity there is a difference between the Americas and Eurasia. Like Ludens said, there always has been some exchange over the Eurasian continent, both of diseases and their respective immunities. Such an exchange was only possible from East to West, because the climate differences weren't as big as when travelling from the North to the South. That's why the exchange between Europe and Africa in the course of history had been much smaller; therfore Europeans travelling to Africa had, untill the 19th century very poor chances to return alive, whereas this was much less a problem for travellers to Asia.
On the Americas there never was a great opportunity for such an East-West exchange, because the main direction of the Continent is North-South. Because of this small exchange, the native Americans had around 1500 a much less developed immune system than the Eurasians. So the Americans died in great numbers, where the Europeans - although hit by local diseases as well - could succesfully conquer and settle the Newe World.

My point regarding our question is: the diseases in Eurasia were pretty well spread, so the consequences of travelling would never have been nearly as dramatic as the events in the Americas. Like Ludens said: mortality wouldn't have been spectacularly bigger for a Greek army in India for example, than it would have been for a Greek army is Greece itself. (I'm not saying it would have been the same, because probably it would have been bigger, but not as spectacular as in your example of the American colonization.)

If you're interested in this whole exchange-of-diseases thing, I can strongly recommend the book "Guns, germs and steel" by Jared Diamond to you. Most of my argument here are taken from there.