In the early 20th century on Earth, when "plastic" ceased being used as an adjective, and instead became the catchall term for polymer-based synthetic materials, it was thought that it would essentially never decompose.  The various microbes that break down other organic materials, did not appear to ever break down plastics.  Plastics were inedible, to microbes, and thus apparently would last virtually forever.  Over time, it became apparent that this was not quite true, but still for a few decades it appeared that, while they did eventually break down from exposure to the elements, into small particles, they did seem to be impervious to rot.

But, each decade that passed, this "well, actually" was revised.  Well, actually, plastics did break down, but very, very slowly.  Plastics did break down, but very slowly.  Plastics did break down, but slowly.

Then came the mid-21st Plastic Plague, and this was finally revised to, "plastics break down".  The rate at which microbes could digest plastics had actually been increasing exponentially for many decades, but from a starting point close to zero, "exponentially" looks much like "unchanging, and very low", for a long time, until suddenly it doesn't.  Then the exponential becomes "large enough to notice", and shortly thereafter it becomes simply "large".

The exact source of the bacteria that could live on polyvinyl chloride, polyethylene, and most other plastics, was never determined for certain.  The leading suspect was the Great Pacific Garbage Patch, a gyre in the mid-north Pacific where discarded packaging material and other sources of garbage, accumulated until it was a sort of plastic soup.  In a region choked with so much of one kind of potential food source, the evolutionary pressure to adapt to be able to feed on it was immense.  But, there were a lot of places on Earth with a lot of plastic; garbage dumps were not far outside every large city.  It could have been any of them, really, and it might be that multiple plastic-eating strains evolved independently in several of them in parallel.  Regardless, by the 2070's, it had become a real problem.

There were also, of course, conspiracy theories about how one government or another, or some other shadowy organization, had maliciously or recklessly developed plastic-eating microbes and then released them into the world.  But it was hard to really find anyone with a proper motive to do so.

It was not until the use of plastics was endangered, that it was properly realized how dependent civilization had become on long-lasting, thermally and electrically unconductive, waterproof and airtight materials.  Every seal, every insulated wire, every spray-on sealant, every place that plastic had been used, and assumed to be not in need of replacing, became a problem that needed a solution.  Or, a substitute.  For about ten years, they simply tried to invent new plastics, that were resistant to the new plastophages, but it turned out that once they had evolved the knack of living on plastic generally, they could adapt to any new polymer rather more quickly.  Plastic became roughly as durable as cloth; even wood lasted longer.

The Colony on Mars had been built during that later stages of denial, and thus the materials shipped to Mars had been subject to extraordinary scrutiny, to prevent any plastophages from hitching a ride.  Most of the Colony was built from metal and glass, but there were a few places (notably the thin polymer sheets sandwiched between glass panels in the Building 3 geodesic domes) where it was used.  Unlike on Earth a few years later, when it had been learned (the hard way) that any place you use plastic, you will need to be able to replace it annually, the few places in the Mars Colony where plastic was used, were not particularly easy to reach (unless you were a microbe).

So, when it was first discovered that Plastic Plague had arrived on Mars, it was perhaps understandable that there was considerable consternation.

"We're doomed," said Elijah, with characteristic vehemence.  "Stench and leakage!  Why would they send polymers that hadn't been sanitized?"

"They were sanitized," said Ophelia, but she sounded uncertain.  "It couldn't have come from us."

"Who else could it possibly have come from?" shouted Elijah.  "It's not like we have a lot of immigrants passing through Mars, you know?!"

"This isn't helpful," said Olivia, frowning, not even looking at Elijah as she thought furiously about what was the best course of action.  She didn't like shouting, or arguing; it made it harder to think of solutions to the problem in front of you.  Her first instinct, in any case where something had gone wrong, was to tamp down on her emotions (and those of everyone else present), to keep them from getting in the way of quickly thinking of the best course of action.

"Isn't it?!" asked Elijah rhetorically, and furiously.

Olivia looked up at him, now frowning in confusion instead of thought.

"Well, no, it isn't," she said.  She turned to Theodore, who had found the evidence of plastophagic infection on the remnants of the rat enclosure.  "How certain are we?"

"Pretty certain," said Theodore, with a grimace.

"Is that why the rats could get out?" asked Ophelia.

"Maybe; it's hard to be sure now," said Theodore.  "They've chewed up that spot pretty good.  I think every rat on the way out gnawed at it a bit more."

There was a long silence, as everyone who happened to be present in Building 3, when Theodore came in to share the news, pondered the implications.

"So, does that mean that the rats could have carried it with them?" asked Ava, as good as ever at thinking about how things could go wrong.  Elijah, who hadn't thought of this possibility until now, gave an exasperated sigh.

"Yes, probably," said Olivia.  "If any bits of plastic were on their fur, or if they ingested any of it while gnawing."

"I knew we should have isolated when we first arrived," said Helene, glaring at Olivia, who blinked in surprise, stone-faced.

"How would that have mattered?" asked Olivia.  "It's plastophages, not a cold virus.  They can last dormant for months."

"Perhaps," said Helene, whose tone made it clear she was not impressed with Olivia's logic.  Olivia, confused, tried to ponder how isolating the newly arrived humans for a week or two could have impacted the contamination of plastophages.  They lived on plastic, after all, not in humans.  It didn't seem to have any bearing on the issue of isolating the Earthlings just after their arrival.

Emma, who was watching all of this in silence, realized that this was one of Olivia's primary weaknesses in her debates with Helene.  Olivia took every statement at face value, as if it were a presentation of a new hypothesis, to be evaluated by comparison against the facts.  But Helene, a more skilled politician, knew better than to give her time enough to do that; she was on to the next point, quickly, before Olivia (or most of her audience, perhaps) had time to ponder whether or not what she had said truly bore on the question at hand.  They both had a vague connection to the concept of "infection", and that was enough to make it seem like a connected point, if you didn't have much time to think about it.

"Do we have an inventory of the places where we've used plastics in the Colony?" asked Helene.  She already knew that they did, but it was a way to quickly move the conversation on.

"Yes," said Olivia, relieved to be back on the ground of rational and relevant conversation.  "We'll call it up to verify, but it's wire insulation in some of the older electronics that dates back to the Founding, the sheets in the geodesic dome panels, some o-rings on water hoses, and a few seals on the vaccsuits that we wear when we go Outside."

"We'll need to get started on making sure we have accurate dimensional files for all of them, and start 3D printing one of each to make sure they're accurate," said Helene.  Olivia nodded; this was all reasonable stuff, she was happy to agree.  Another mistake, thought Emma; she let Helene imply that Olivia had made a mistake earlier, and now she's agreeing to her suggestions; it gives everyone the impression Helene is always correct, and back in charge.

Which meant, thought Emma with a bit of a wince, that perhaps she was back in charge.