It seems that the "Covenant" crew shares the same foolhardiness as the crew from the previous "Prometheus" movie: they descent on an unknown planet without any protective equipment other than rifles.

Even on Earth there are thousands of plants, insects, viruses and bacteria (not counting other dangers) that can kill you when they get in contact with your skin (or, gods forbid, - when you inhale them), and yet the crew decides to land in the middle of the forest dressed as if they were heading for a family hunting trip.

I understand that this is a plot device, but is there an in story or other official explanation for such carelessness?

Edit: To clarify, I'm looking for an in-universe explanation, even something as thin as "Thanks to those super-pills we've taken at home we should be immune to all infections"