>>521096487 (OP)
There are two kinds of life: Aerobic, the kind that thrives at high oxygen levels, and anaerobic, which prefers less oxygen and more methane, ammonia, etc. Anerobic life had its time, but the expansion of forests and the concomitant high-oxygen-based creatures pushed the anaerobic life into smaller niches.
Humans are unaware of the neurological influences of the anaerobic entities hiding among their cells. Enough microorganisms all working together can collectively act as a much larger intelligent life-form. Since they are genetically identical, their DNA coils oscillate at exactly the same set of frequencies while being on the same wavelength, giving them telepathic linkage. Their goal is to eventually ruin this planet's oxygen level and render it uninhabitable for humans and other life-forms.
Most people have a severe oxygen deficiency, just as any mammal's immune system uses a lot of single-atom oxygen to handle germs. Our atmospheric oxygen levels are at an all-time low and the oxygen pressure in our blood is insufficient to protect us from anaerobic microbes. Your body is meant to be at least 80% water, which is nine-tenths oxygen, and you're supposed to be composed of two-thirds oxygen if not more, twice as much as everything else in you combined.
All disease organisms prefer much lower oxygen saturations than what your cells function best at. Their primitive, fragile membranes break down at the concentrations of a healthy human cell. Oxygen is the only thing you can die in minutes without. Get enough oxygen and it protects you from disease.