The dust got into everything; wrinkles in the rubber gaskets, the edges between a helmet and a visor, under the plastic joints around an elbow or between the proximal and distal phalanges of a thumb. Everything. It doesn’t matter how much compressed air you got blasted with in the airlock or if you were diligent in keeping your exosuit double-sealed when it was stowed. That fine brown dust–which looks a lot more like chocolate milk powder than rust–gets into the air of the habitat. Of course, it couldn’t be avoided, which was why there was an elaborate air purification system to begin with. Ironically enough, that’s where it started; the air purifiers. Not that anyone knew it.
The filters caught all the usual contaminants: pollen from the farming modules, dander from the farmers themselves, and dust mites that made the interplanetary journey with those same farmer’s personal effects. Not to put the blame on the farmers, or anyone really. The filters removed airborne mold, because mold grows everywhere that you don’t look for it. They pulled the stink from the latrine with porous layers of carbon. And the filters trapped the dust. They were designed to trap dust, so no one thought twice about it; the air purification system was apparently working.
But the system didn’t just snatch up contaminants; it attracted them with a static charge, and it irradiated them with ultraviolet rays. These methods introduced energy into a filter that was home to organic compounds and Martian dust. The same dust left over from the biologists’ active search for ancient microbial life. Their tests were looking for the wrong compounds, but they were at least looking in the right place. What they had inadvertently carried into the habitat was unlike anything on Earth–it didn’t have DNA, or even RNA, but its closest analog would still be a virus. And the closest analog to the contaminated air purification filters was now a Petri dish.
It took a long time for people to return, and once they did they couldn’t determine the cause beyond a reasonable degree of certainty, so they left again, after a prolonged quarantine, of course. You see, the last of the original colonists had taken to burning the bodies–an understandable precaution–but the smoke damage erased all evidence of the multigenesis that had taken place all around them, in the walls, and under the floors, and in front of their eyes. An understandable precaution, but a useless one. They didn’t last much longer.
After that, people set their eyes to Europa, and then the stars. It was a big universe; we didn’t need Mars. If we weren’t welcome there, it was better to leave it alone.