The question of ‘how we are going to avoid this when humans finally land on Mars’ has only one possible answer; we might have already determined the presence of life there. Hence, cross contamination will not be that big of an issue, though they will still take precautions to avoid disturbing the balance of nature on the planet. The suits of the exploration teams are sterilized before leaving their habitat. Any material collected on the planet or another extraterrestrial body is sealed in sterilized containers before bringing them inside.
These scientists and astronauts wear suits in the clean rooms while experimenting and designing the space mission on earth. These are designed to have a low particle count in the air. Otherwise, it may mess up the experiment and equipment, thus jeopardizing the entire mission.
Do you have any other theory about these fancy suits of spacemen? Let ’em rip in the comments’ section!