Why do rodents often have more fleas than animals without permanent homes?

Rodents are the most heavily parasitized group of mammals because their habit of building nests in holes provides an ideal environment for the development of flea larvae.1

The flea life cycle—consisting of egg, larva, pupa, and adult—is supported by the organic debris, such as dried skin and blood, found within these nests. Because the larvae require this specific habitat to thrive, animals that do not build permanent homes or nests tend to have fewer fleas.21