Information about demodex....
Demodectic Mange
From that site:
Demodectic mange, also called "demodicosis," is caused by a microscopic mite of the Demodex genus. Three species of Demodex mites have been identified in dogs: Demodex canis, Demodex gatoi, and Demodex injai. The most common mite of demodectic mange is Demodex canis. All dogs raised normally by their mothers possess this mite as mites are transferred from mother to pup via cuddling during the first few days of life. Most dogs live in harmony with their mites, never suffering any consequences from being parasitized. If, however, conditions change to upset the natural equilibrium (such as some kind of suppression of the dog's immune system), the Demodex mites may "gain the upper hand." The mites proliferate and can cause serious skin disease.