Whether or not they knew they were transporting disease at the time really doesn't excuse the end result: mass genocide. And of course this doesn't authorize taking land.
As for lands, I don't know, lands all over the world have been changing hands since human beings learnt how to walk, most of the time after violence/war. It happened in 1500 when the concept of "nation" didn't even exist and the only way to say that a land was yours was to be able to keep it.