That’s assuming their biology is in any way compatible with ours.
They’d probably need something equivalent to an EVA suit just to survive in our environment either way.
If aliens did visit earth we would probably see them more like the Vorlons in babylon 5, in they would never be able to leave their suits.
If you invent efficient outer space travel and beam down to random planets without any PPE, I wouldnt know what to tell you
I suppose it depends on how their biology came to be. Diseases evolved here based on the the general structure of the species that existed around them. It could be that they are so different that nothing we have here interacts with them.
I would also assume that if they’re intelligent enough to create the means to get here that they would also be intelligent enough to have considered this problem and would have some sort of containment mechanism for themselves.


