Fleas are small, flightless insects that form siphonophores. As external parasites of mammals and birds, they live by consuming the blood of their hosts. Adults are about 3 mm (0.12 in) long and usually brown. The body is flattened laterally so that they can pass through the fur or feathers of the host. Strong claws prevent them from being expelled. They don't have wings, the mouthparts are suitable for piercing the skin and sucking blood, and the hind legs are suitable for jumping. The latter enables them to jump about 50 times the length of their body, second only to the leaping feat of frogs. The larvae are vermicular and have no limbs. They chew mouthparts and eat organic debris.

More than 2500 fleas have been described worldwide. Siphonophores are most closely related to the snow scorpion (boreidae), which is placed in Mecoptera.

Fleas originated in the early Cretaceous, are most likely exoparasites of mammals and marsupials, and then transferred to other populations, including birds. Each flea is more or less an expert on its host animal species: many species have never propagated on any other host, although some of them are less selective. Some flea families are unique to a single host group: for example, the family Magnoliaceae is only found in armadillo, the family heterosheath is only found in bats, and the family bizarrepidae is only found in elephant sh. Xenopsylla cheopis is the carrier of Yersinia pestis, which is the bacterium causing plague. The disease is spread by rodents such as black mice, which are bitten by fleas and then infected by humans. Major outbreaks include the Justinian plague and the black death, both of which have killed a large part of the world's population.

Fleas appear in various forms in human culture, such as the flea circus, John Donne's erotic poems such as "flea", music works such as Maud musbosky and Charlie Chaplin's films.