Mites hide in your bed and breed on your face. They’re smaller than the period at the end of this sentence.

Rob Dunn in National Geographic:

Several years ago I made a bet about face mites, animals that live in hair follicles. They are so small that a dozen of them could dance on the head of a pin. They are more likely, though, to dance on your face, which they do at night when they mate, before crawling back into your follicles by day to eat. In those caves mother mites give birth to a few relatively large mite-shaped eggs. The eggs hatch, and then, like all mites, the babies go through molts in which they shed their external skeleton and emerge slightly larger. Once they’re full size, their entire adult life lasts only a few weeks. Death comes at the precise moment when the mites, lacking an anus, fill up with feces, die, and decompose on your head.

Currently two species of face mites are known; at least one of them appear to be present on all adult humans. My bet was that even a modest sampling of adults would turn up more species of these mites, ones that are totally new to science.

Biologists often make bets; they call them predictions to sound fancier. My bet was based on an understanding of the tendencies of evolution and of humans. Evolution tends to engender its greatest richness in small forms. Humans, on the other hand, tend to ignore small things. Aquatic mites, for example, live in most lakes, ponds, and even puddles, often in densities of hundreds or thousands per cubic meter. They can even be found in drinking water, yet few people have ever heard of aquatic mites, including, until recently, me. And I study tiny things for a living.

More here.