Context: Humans have escaped a a super-bacteria, similar in situation to the Bubonic Plague, that they could not create a cure for. They've done this by engineering gargantuan trees and adapting to lower oxygen levels.
However, I'm not sure my explanation of "the bacteria cannot survive in lower oxygen levels" makes sense because, well, trees produce oxygen, and it's supposed to be super-adaptive.
What's a plausible reason a bacterial plague couldn't survive in higher atmospheric conditions, or could only exist closer to the planet's surface? Open to any changes here including changing disease type. If there isn't one, I'll work on switching up the premise.
Edit for additional information: The planet itself is much larger than our own, and the layers of the atmosphere are wider to match. The original idea was that the bacteria somehow could not mutate to live in even marginally lowered oxygen levels, I'm talking the difference between sea level/airplane cruising altitude here. Worldbuilding-wise there is a heavily monitored quarantine layer between society and freefall to the surface. It's clear I need more research in general on atmospheric layers and oxygen production, the question is just if there's a different reason a bacteria could only survive close to the surface of a planet. Unfortunately I don't have numbers to give you, but continuing an Earth parallel the trees themselves would need to be at least 30,000ft.
Edit 2: The trees are genuinely actual wood trees. The question of how they grew them is currently not something I need scientifically answered for various worldbuilding reasons, or at least it's not the focus here. Assume these insanely massive trees are just a given.