Even then it would have extremely struggled to handle 30+ million new cases of Small Pox, TB, Bubonic Plague, etc.
Luckily as far as modern technology can show we're extremely certain Mars has no life we'll accidentally genocide while exploring/terraforming.
Indians had no trouble crisscrossing America.
Well yeah there weren't any human infectious illness where... there aren't any humans.
This video explains the history of man kind's infectious illness very well https://www.youtube.com/watch?v=JEYh5WACqEk
A single smallpox laden explorer would have started a plague in the Americas and a single native American would likely have died of one of the many plagues in Europe that Europeans had grown up with (and still often died from).
The problem of safety is not always placed on the explorer, sometimes the explorer is the danger to his environment.