A group of small flattened insects with considerable jumping powers. The adults are obligate parasites of warm-blooded vertebrates.
Most species are restricted to one or a few related host species, or species with similar habits. But (as any cat owner knows) displaced fleas are attracted to any warm-blooded animal, so any species might occasionally be found on an unexpected host.
The worm-like larvae live among detritus in nests. For this reason fleas are restricted to animals that build nests or, at least, return to the same home or burrow - Ungulates (cattle, horses, deer etc) do not have fleas.