Chytrids are unloved microbes. One kind of chytrid, Batrachochytrium dendrobatidis, is destroying the worlds of amphibians. Hundreds or thousands of species of frogs, salamanders, and legless amphibians called caecilians are on the brink of extinction as a result. Other chytrid species perform critical ecological functions—some break down chitin, the hard material in the exoskeleton of insects, and others live in the hind guts of ruminants, where they help digest cellulose, a molecule in dead plant matter that is hard to break down. Chytrids generate spheres nested within other spheres, clear bubbles containing darker bubbles. When living in microbial ecosystems,...