I know that this sounds weird, but there is lots of evidence suggesting the introduction of necessary microbes that make our life possible here, on Mars, for example, a paper published a couple of moths ago in the journal FEMS Microbiology Ecology argues that the first colonists of the Red Planet should be microorganisms, the bacteria, viruses, and fungi that make possible many of life’s processes here on Earth, such as digestion or decomposition, are completed by bacteria.
This however doesn’t mean that we would just send a spaceship full of microbes and bacteria that might be harmful for humans, what would happen, is that we would make lots of research and experiments to determine what we send, we will of course not send highly infectious diseases, (such as Coronavirus), for obvious reasons, this if we ever send anything at all.
Another thing that we might do, is to send some super resistant bacteria, knowing how harsh temperatures and conditions tardigrades survive, it shouldn’t be too hard to find beneficial bacterias which might survive the extreme conditions that life on Mars faces.
The easiest, most simple way of colonizing other planets, would be to send super-resistant microorganisms into the atmosphere of those planets, and just hope they evolve into intelligent life, the first colonists of Earth were just some unicellular organisms or bacteria, and they did a pretty good job, didn’t they?
As there aren’t many known diseases on Mars, there is the risk of death which would hold back missions the other astronauts would need to go on. Microgravity is not a disease, but it is known to cause health problems, such as muscle loss and bone demineralization. The risks of going to space are high, but it’ll be worth it if we manage to colonize other planets.
Luckily when the astronauts do come back, we will do tests on the astronauts to check for any kind of anomaly or disease, there are machines that are in work to check if they have the new diseases that they might have originated on Mars.