I find it unrealistic for any movie involving contact with aliens to not even consider how our and their microbial life would interact with each other. There should be a default rule of "oh you touched the alien? Ok, you will be the quarantine subject that we will monitor to make sure you haven't caught some slow developing disease that will inevitably kill any earth life in your vicinity because we haven't developed any kind of resistance to it".

Except instead of "touched the alien" it would really have to be "entered their atmosphere with your ship" because the ship itself could bring back contaminants that would eventually destroy all life on our planet. And maybe even entering the atmosphere isn't needed if microbial life occasionally gets swept into space by extreme weather, meteor interactions with the atmosphere, maybe solar ones also. Including this because it's plausible that earth life originated elsewhere and was just seeded here by some event that allowed life to escape some other location and survive to come here. We've also had ships return to earth with life detectable on their outside--because someone on earth sneezed on it before it was launched and the microbial life wasn't destroyed by being in space.

Similar line of thought when they go on the alien planet and just breathe the atmosphere because they detect oxygen.