There are 18 families and 2,000 species of fleas. These brown, shiny, and wingless insects have tough, laterally flattened bodies covered with backward-pointing spines and bristles. The enlarged hindlegs are part of a unique jumping mechanism involving energy storage in rubberlike pads of protein. Fleas have specialized mouthparts for sucking blood. They are ectoparasites, living on the outside of a host animal and feeding on it without killing it. Less host-specific than parasitic lice (here), some fleas are found on over 30 host species--mostly terrestrial mammals but also birds. The larvae do not suck blood, but scavenge on the excrement of adult fleas, detritus, and dried blood.
 

This family looks typical of its order and may have bristle-combs on its pronotum and cheeks. Common fleas are extoparasitic on humans and a wide range of other mammals, including dogs, cats, and rabbits.
 

Eggs are dropped in hosts' nests or burrows. Adults can survive for a long time without a blood meal. Emergent fleas remain in their cocoon until they sense a host's presence.
 

Worldwide. On mammalian hosts in a wide range of habitats.
 

Many species spread disease. The Dog Flea carries a tapeworm that affects dogs, cats, and humans. The bacterium that caused bubonic plague in medieval Europe was carried by various types of rat flea.