Fecal Transplants.

Yes seriously.

It's exactly what you think. Healthy person's poop in a blender, pour into a bag, enema. (Or, if you can hold your nausea, a nasal tube that goes directly to your stomach, to hit the upper digestive tract too).

If it works, it works fuckin' instantly. You now have a healthy person's guy biome.

Also, an instant cure for c-diff poisoning and a bunch of other stuff.

Probably the lowest tech, most massively impactful treatment we've discovered for tons of stuff.