Human visitors to Mars could bring along numerous microscopic colonizers. That's both good and bad.
Mihai Andrei by Mihai Andrei February 22, 2021
In a new study, NASA and German Aerospace Center scientists found that Earth microbes can withstand Martian conditions, which means we could use them there, but they could also pose risks for astronauts.

Sturdy microbes
Youre never really alone. Youve got a gazillion tiny critters on yourself at any given moment. Most are harmless. Some can be useful and of course, some can be harmful.
Try as we might (and space agencies do try), theres no realistic way to eliminate all microbes from a crewed mission. Even with all the available decontamination procedures, you cant really kick all off them from everywhere. If (or perhaps when?) we fly astronauts to Mars, the mission will bring some microbes along, like it or not.
Some have suspected that this wouldnt matter at all, because the microbes just wouldnt be able to survive on Mars. But a new study says otherwise.
More:
https://www.zmescience.com/science/microbial-earthly-life-could-survive-on-mars-temporarily/