Adult fleas are about 1/8 inch long and flattened laterally (up and down). They are brown colored, but mated females carrying eggs have a larger, lighter-colored belly. They become reddish brown after a blood meal. Fleas have powerful hind legs equipped for jumping. Various species differ slightly in structure. Fleas have complete metamorphosis, meaning they have a larval stage and a pupa from which the adult emerges. Fully grown larvae are about 1/8 to 3/16 inches long.

Cat fleas are the most common domestic species of flea. Adult fleas spend their entire lives on the host, but eggs and larvae develop in the environment of the host. Both male and female fleas feed on blood. After mating and a blood meal, female cat fleas lay 4 to 8 eggs, which remain loose within the animal's fur. Eggs, along with adult fecal matter (dried digested blood), are dislodged and dropped into pet bedding or the area where the animal sleeps. Eggs hatch within 2 days (range is 2 to 12 depending on temperature). Larvae feed on organic debris but require dried blood as a part of their diet. Flea larvae can move around their environment. They require high relative humidity to survive, and development to the pupa can take a week to several months. The pupal cocoon has debris from the environment incorporated to conceal it. With hosts available, the adult flea will emerge within 4 to 14 days. If no host is present, the pupa can remain in a preemergence' stage for several months. Pupae are resistant to pesticides. Adult fleas will feed by their second day after emergence, but they can also wait for weeks to find a blood meal.

The most obvious damage caused by fleas is their itchy bite. Fleas have preferred hosts but can be flexible in choice when necessary. Cat fleas utilize a diverse range of hosts, including dogs, rodents, raccoons, and other urban wildlife. They will bite humans readily. Cat fleas are an intermediate host for dog tapeworm, transmitting this parasite to dogs and cats when the fleas are ingested. There is evidence that they also transmit murine typhus and may transmit plague. Although mainly associated with pets or wildlife, fleas can become a problem in structures when no animals are present. Their superior jumping ability enables fleas to hop onto shoes or pants of a passerby to gain entry into a building. Further. more, when a flea-infested structure is vacated, fleas can persist for 6 or more months. When people or animals return to the site, adults emerge from dormant pupae seeking a host and blood meal.