Fleas are small, agile, usually dark colored, wingless insects with tube-like mouthparts adapted to feeding on the blood of their hosts. Their bodies are laterally compressed permitting easy movement through the hairs or feathers on the host’s body or in the case of humans, under clothes.
Fleas attack a wide variety of warm-blooded vertebrates including dogs, cats, humans, chickens, rabbits, squirrels, rats, ferrets, and mice. Fleas are a nuisance to their hosts, causing an itching sensation, which in turn may result in the host attempting to remove the pest by biting, pecking, scratching, etc. the vicinity of the parasite.
Fleas are not simply a source of annoyance, however. Some people and animals suffer allergic reactions to flea saliva resulting in rashes. Fleabites generally result in the formation of a slightly raised swollen itching spot with a single puncture point at the center (similar to a mosquito sting). The bites often appear in clusters or lines of two bites, and can remain itchy and inflamed for up to several weeks afterwards. Fleas can also lead to hair loss as a result of frequent scratching and biting by the animal, and can cause anemia in extreme cases.
Besides the problems posed by the creature itself, fleas can also act as a vector for disease. For example, fleas transmitted the bubonic plague between rodents and humans. Typhus fever, and in some cases, tapeworms can also be transmitted by fleas.