Adult fleas are not only a nuisance to humans and their pets, but can cause medical problems including flea allergy dermatitis, tapeworms, secondary skin irritations and, in extreme cases, anemia. Although bites are rarely felt, it is the resulting irritation caused by the flea salivary secretions that varies among individuals. Some may witness a severe reaction like general rash or inflammation resulting in secondary infections caused by scratching the irritated skin area. Others may show no reaction or irritation acquired after repeated bites over several weeks or months. Most bites usually found on the ankles and legs may cause pain lasting a few minutes, hours or days depending on ones sensitivity. The typical reaction to the bite is the formation of a small, hard, red, slightly raised itching spot. There is a single puncture point in the center of each spot. Ants and spiders leave two marks when they bite. Mosquitoes, bees, wasps and bedbugs cause a large swelling or welt. Also, fleas may transmit bubonic plague from rodent to rodent and from rodent to humans. Oriental rat fleas can transmit murine typhus which causes endemic typhus fever among rats and from rats to humans. Tapeworms normally infest dogs and cats but may appear in children if parts of infested fleas are accidentally consumed.

Adult fleas have dark reddish brown, wingless, hard bodied which is difficult to crush between fingers, have three pairs of legs, hind legs enlarged enabling jumping and are flattened vertically or side to side allowing easy movement between the hair, fur or feathers of the host. Fleas are excellent jumpers, leaping vertically up to seven inches and horizontally thirteen inches. An equivalent hop for a human would be two hundred feet vertically and four hundred fifty feet horizontally. They have piercing sucking mouthparts and spines on the body projecting backward. Also, there is a row of spines on the face known as a genal comb. Both spines are about the same length in the cat flea. The rabbit flea has a vertical genal comb with blunt spines. The genal comb is absent in both rat fleas. Eggs are smooth, oval and white.

Fleas pass through a complete life cycle consisting of egg, larva, pupa and adult. A typical flea population consists of fifty percent eggs, thirty five percent larvae, ten percent pupae and five percent adults. Completion of the life cycle from egg to adult varies from two weeks to eight months depending on the temperature, humidity, food, and species. Normally after a blood meal, the female flea lays about fifteen to twnty eggs per day up to six hundred in a lifetime usually on the host like dogs, cats, rats, rabbits, mice, squirrels, chipmunks, raccoons, opossums, foxes, chickens, including human. Eggs loosely laid in the hair coat, drop out most anywhere especially where the host rests, sleeps or nests like rugs, carpets, upholstered furniture, cat or dog boxes, kennels, sand boxes, etc.. Eggs hatch in two days to two weeks into larvae found indoors in floor cracks and crevices, along baseboards, under rug edges and in furniture or beds. Outdoor development occurs in sandy gravel soils like moist sand boxes, dirt crawlspace under the house, under shrubs, etc. where the pet may rest or sleep. Sand and gravel are very suitable for larval development which is the reason fleas are erroneously called sand fleas.

Larvae are blind, avoid light, pass through three larval instars and take a week to several months to develop. Their food consists of digested blood from adult flea feces, dead skin, hair, feathers, and other organic debris. Larvae do not suck blood. Pupa mature to adulthood within a silken cocoon woven by the larva to which pet hair, carpet fiber, dust, grass cuttings and other debris adheres. In about five to fourteen days, adult fleas emerge or may remain resting in the cocoon until the detection of vibration like pet and people movement, pressure in host animal lying down on them, heat, noise, or carbon dioxide, meaning a potential blood source is near. Most fleas overwinter in the larval or pupal stage with survival and growth best during warm, moist winters and spring.

Adult fleas cannot survive or lay eggs without a blood meal, but may live from two months to one year without feeding. There is often a desperate need for flea control after a family has returned from a long vacation. The house has been empty with no cat or dog around for fleas to feed on. When the family and pets are gone, flea eggs hatch and larvae pupate. The adult fleas fully developed inside the pupal cocoon remains in a kind of limbo for a long time until a blood source is near. The family returning from vacation is immediately attacked by waiting hungry hordes of fleas. In just thirty days, ten female fleas under ideal conditions can multiply to over a quarter million different life stages.