There’s a risk both ways. Earth microbes could contaminate Mars and Mars microbes could contaminate Earth. This is the risk of forward and backward contamination.

If there is Mars life and humans have landed on Mars, then you would think that surely before then they have studied this life and found out that they will not make it extinct (or that it is so uninteresting for some reason that it makes no difference if it goes extinct) and it is no harm to us.

However the problem here is that humans can be very impatient and they might land on Mars without first investigating to find out what life there is there and if Earth life is harmful to Mars or vice versa. We actually have it as a requirement under international law to protect Mars and other places in the solar system from Earth life and vice versa. All our space agencies abide by this ruling, and we have international meetings every two years of thousands of scientists to examine it carefully and develop guidelines.

Joshua Lederberg, Nobel prize winning microbial geneticist was amongst the first to draw attention to the issue of back contamination. So, yes, it’s not likely that Mars microbes are adapted to infect humans. But on the other hand, humans are not adapted to defend ourselves against Mars microbes. Our defense systems work by looking out for particular chemical signatures of life. But what if Mars life doesn’t have those signatures? It might seem no more harmful to our bodies than an artificial implant. That’s especially possible if it has a different biochemical basis from Earth life maybe without proteins, DNA, etc.

Then also diseases can leap to higher animals from microbes. As an example, legionnaires disease is a disease of amoebae that uses the same mechanism to infect humans. So similarly, Mars microbes adapted to infect other Mars microbes could perhaps also harm us.

Then also if the life is related to Earth life, even billions of years ago, then it could engage in horizontal gene transfer. Happens very readily overnight in sea water. The result would be microbes with mixed capabilities of Earth and Mars life.

However there are many other ways microbes from Mars could be harmful. They could cause no direct harm to humans but harm our crops, plants, animals. They could harm our ecosystems or the oceans.

I think one of the best examples myself, just to get you thinking about the possibilities, is a photosynthetic lifeform. Life on Earth does photosynthesis in three main ways - involving oxygen, involving sulfur, and the other way is to use a proton pump, this uses light to create a proton gradient which is used to produce chemical energy in the form of ATP which the organism can then use as a source of energy.

So what if Mars has life based on some fourth form of photosynthesis and then this life is returned to Earth? It might take it a while to adapt to our oceans or it might do it rapidly. Anyway now suppose that this form of photosynthesis is more efficient n some way than the photosynthesis we have already? Then it outcompetes the green algae and other photobionts in the oceans.

And suppose it is inedible to sea life, or is even poisonous, creates chemicals that poison Earth life. Doesn’t have to be adapted to it, to do that, e.g. green algae produce BMAA which is implicated in Alzheimers. No advantage to the green algae to do that. Just got an accidental similarity with L-Serine, gets misincorporated and causes protein folding problems.

In this way a small microbe could disturb an entire ecosystem - all sea creatures that depend on the algae and photobyonts. Then also all creatures that eat them, and so on.

That’s just an example to get you thinking, to realize that we can’t really hope to anticipate all possibilities here. There is no substitute for actually knowing what is there.

I think we just should not send humans to Mars until we know what we are dealing with - in either direction. And we should also be very careful about returning samples from Mars. Instead of returning them to the Earth surface - why not return them to a high orbit above Earth, above GEO say? Then study them there in a preliminary way. If there is no life, sterilize just to be sure and return to Earth swiftly. If there is life, then study it in orbit until we know what we are dealing with and then take the appropriate precautions - or maybe not return it at all. We can always break off smaller samples and sterilize them and return them to Earth for the geologists.

Anyway for more about this, see my

Why quarantine won't work - protecting Earth, and humans sent to Mars, from Mars life (if it exists)