Thanks, @bobydimitrov
I went to ask for some advice from the people that run the organic farm not so far from my house, and they said it is probably leaf spot disease. A bit of a bummer, because there aren't any biological ways to treat it.
For now, I'm trying to keep it from spreading by spraying every inch of every plant with a thin layer of lava rock and adding lava rock to the soil.
For now, it seems to be working. I know it has only been a couple of days, but some plants that were infected and of which I removed the sick leaves seem to grow healthy new leaves.
I'm crossing my fingers!
I'll definitely read up about the fungus that eats other fungi... interesting 👍
