Fleas are bloodsucking parasitic insects that live on pets, livestock, and humans. Human fleas (Pulex irritans) are relatively uncommon, and the species most likely to bite humans is the cat flea (Ctenocephalides felis) or the rat flea (Xenopsylla cheopis).

Fleas are small, 1-4 mm in length, and blackish-brown. Although wingless, fleas are still capable of taking giant leaps, jumping to a height of several meters. Adult fleas feed only on blood and are capable of living long periods without feeding. Fleas most often bite people around the legs and the ankles. The flea injects antigenic saliva after piercing the skin with its mandibles. The bites, which are felt immediately, become increasingly irritated and may remain sore for as long as a week. Itching may be generalized or just at the site of the bites.

Flea bites produce a variety of effects, ranging from a transient wheal to prolonged symptoms that may last for years depending on the sensitivity of the person bitten. Children aged younger than 10 are generally more sensitive than older people, as people tend to become desensitized by repeated exposure over time. Flea bites in children produce a papular urticaria in which recurrent or chronic pruritic papules occur on the exposed skin areas. Patients with hypersensitivity may also develop bullous reactions.