So I was thinking about War of the Worlds a little bit, but then reversed it for if humans ever did hypothetically get to other worlds.
So if we ever were to find a planet with breathable atmosphere, wouldn't it be an incredible risk of even attempting to live there, because of micro-organisms.
Any planet we step on could be like the Native americans coming in contact with Smallpox, or even a worse, more alien/dangerous version of a Virus, or bacteria which could potentially wipe out any colonizing party, since there's no natural immunity.
I'm guessing science would be so advanced at that point to rapidly come up with some sort of immunization, but since there's billions of different species of micro-organisms, it would probably take decades of research on that planet to fully immunize a human being to the point where they can actually even survive on the planet.