A recent study, reflected in numerous news articles, indicates that the surface of Mars may be more toxic than originally thought.
The short version is as follows:
We already knew that perchlorates exist in may locations in the upper layers of the surface of Mars. We knew that iron oxide was common We knew that the sun's ultraviolet light was mostly unfiltered when it hit the surface.
What the study showed was that these factors in combination are 10.8 times better at killing ordinary bacteria than ultraviolet light alone.
This is actually a good thing for human exploration of Mars. A key issue as we approach the time of humans traveling to the red planet will be planetary protection. Humans have ten bacterial cells in our bodies for every human cell, and we shed them constantly. It's going to be very difficult to keep the spacesuits clean and in good condition without human contact. The only thing that could make planetary protection a non-issue for early missions is a planet with the ability to sterilize itself at the air and surface dust level. That is exactly what Mars can do, and it does it over ten times better than we expected.
That said, a key factor in settlement is getting under a blanket of radiation protection such as soil or ice. If we confined mining operations to A) robotic excavators and B) avoided human interaction with subsurface zones that could touch a water table, we wouldn't have the issue of forward contamination (or back contamination, should life exist there). This can be managed if one keeps habitats on the surface and buries them in the ice-filled equivalent to sand bags. This provides better cosmic ray protection, easier construction, and avoids the issue in both directions until the issue of life on Mars is resolved.