Hi,
when the Native Americans got into contact with Europeans a significant amount of them died just because they were not resistant to the different diseases, which were brought by the Europeans. Now you can't really see the effect because the world is obviously more connected and people travel constantly.
However image the same scenario with Mars, people there could develop bacteria that could be dangerous to us.
What do you think? (I assume a long period where people will not travel back and forth.)
