We can't really survive somewhere that we're not adapted to live in.
Actually, I already survive somewhere that humans are not biologically adapted to. When I go outside during winter, I could die. But I overcome this with technology. When the temperature drops, I wear a coat.
Why is wearing protective gear (space suits, encounter suits, whatever) a showstopper for alien visitors? For that matter, why couldn't they alter their biology to match the environment? We are much closer to modifying humans than to visiting the stars (some Chinese researchers are already trying to create HIV-resistant babies.) Being able to modify one's biology significantly may even be a prerequisite to surviving interstellar travel.
There is also the possibility that the alien visitors are so alien, that our microbes wouldn't affect them at all. If they were all robots, for example.
...yeah. Actually, forget everything I just said, and let's go with that. The aliens are all robots.
Aliens are all robots is the most feasible.
And yeah, I'm well aware of the Chinese CRISPR babies. Thing is that comes with a lot of ethical problems for a reason. Not the least of which being that straight Mendelian genetics is just scratching the surface and altering one gene could disrupt multiple gene networks. I'm not a computer programmer, but I'm sure a computer programmer can easily spot an analogy here. It of course depends on exactly what's being done in genetic modification, but if you're so heavily modifying a genome to be able to handle alien worlds, those aliens might be aliens to the aliens that point