Fleas are familiar to most people with a dog or cat. They are the small, flat and normally hidden insects whose quest for blood sets the animal to frantically snapping at its tail or scratching its ears. Cat (and dog) fleas are the most familiar of well over 1,800 species and subspecies. A few other species have been instrumental in spreading serous human diseases, such as bubonic plague. Others pass diseases onto animals and a few burrow into the skin of their hosts.

Fleas belong to the order Siphonaptera and are subdivided into three superfamilies. Pulicoidea, with about 25 genera, contains most species of medical and veterinary importance such as the cat, Oriental rat and sticktight fleas. Certopsylloidea is the largest superfamily with 150 genera that are mostly Neotropical in distribution in southern Mexico, Central and South America, and the West Indies.

The best known to North Americans is the cat flea (Ctenocephalides felis), which pursues cats and dogs. The very similar dog flea (C. canis) is far less common. These fleas jump onto their hosts and remain there.

Few other flea species are familiar, but the Oriental rat flea (Xenopsylla cheopis) which carries the causative bacterium for the plague, retains notoriety for several hundred cases occurring in wild rodents in the U.S. over the last several decades.