Germicidal UV could make airborne diseases as rare as those carried by water

There is a giant orb in the sky that emits UV and has for billions of years.

I think the microbes are still trying to figure this one out.

You are correct, however most of the harmful rays get filtered out in the upper atmosphere. Far-UV doesn't reach Earth, only UV-A and small amounts of UV-B (if the ozone layer is more or less intact that is!).

> I think the microbes are still trying to figure this one out.

They mostly figured it out a couple billion years ago. Cyanobacteria oxidized Earth's surface until the atmosphere was flooded with molecular oxigen, that gets turned to ozone in the stratosphere, filtering most UV. Pretty large engineering feat for a bunch of microbes.