Pulex irritans, the human flea, is one of more than 1,600 species and subspecies of fleas that populate the Earth from the Arctic Circle to the deserts of Africa. Fleas belong to the insect order Siphonaptera and parasitize mammals and birds for their blood, using specialized anatomical structures to attach to the hosts' skin.

As its name suggests, the preferred food for Pulex is human blood, but it will feed on other mammals as well. Historically, human populations have had ongoing infestations of Pulex, but they still occur in some human populations. Infestations of Pulex are less of a problem in societies where personal hygiene dictates frequent bathing and clothes laundering.

The human flea is not the primary species responsible for transmitting the bubonic plague throughout Europe during the Middle Ages (that dubious honor belongs to the rodent flea), although it is capable of spreading it. One disease that Pulex is known to transmit is murine typhus, a mild form of typhus caused by the bacterium Rickettsia prowazekii. The flea is infected by feeding on a human who has the disease; the bacteria grows in the epithelial cells lining the flea's gut wall and are excreted in the insect's feces. The infection kills the flea after 12 to 18 days. A person becomes infected by scratching the flea bite, thus rubbing the flea's infected feces into the wound. After an incubation period of one to two weeks, an infected person experiences headaches, loss of appetite, malaise and a rapid rise in temperature with fever, chills, marked prostration and nausea. Four to six days later a rash appears over most of the body. Aside from the possibility of transmitting disease, the human flea's saliva contains enzymes and histamine-like substances, which cause allergic reactions in their victims.

The flea's life cycle consist of four stages: egg, larva, pupa and adult. Eggs are laid on the body of the host animal, but since they aren't sticky they can drop in any number of places. The larva resembles a small legless caterpillar and it feeds on dried excrement, dried bits of skin, dead mites, dried blood, and other organic debris. Fecal matter from the parent flea is essential to the successful metamorphosis of some species of flea larvae. During this time the parent flea consumes a great deal of blood, up to 30 times its own weight, to produce a large quantity of feces for its larvae. After three molts, the larvae spin a silk cocoon that begins the pupal stage. The pupae emerge as adults days, weeks, or even months later. If conditions are unfavorable, a cocooned flea can remain dormant for up to a year, waiting for warm-blooded creatures to prey upon. At any given time, only five percent of living fleas are in adult form, while most are in the egg, larva, or pupa stages. The life span of adult fleas varies from several weeks to over a year.