Fleas

Fleas are small (1/16 to 1/8-inch (1.5 to 3.3 mm) long), agile, usually dark colored, wingless insects with tube-like mouth-parts 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. Their legs are long, the hind pair well adapted for jumping (vertically up to seven inches (18 cm); horizontally thirteen inches (33 cm)[2]) - around 200 times their own body length. The flea body is hard, polished, and covered with many hairs and short spines directed backward which also assists its movements on the host. Its tough body is able to withstand great pressure, likely an adaptation to survive attempts to eliminate them such as scratching. Hard squeezing between the fingers is normally insufficient to kill the flea.  It may be necessary to capture them with adhesive tape, crush them between the fingernails, roll them between the fingers, or put them in a fire-safe area and burn them with match or lighter. They can also be drowned.

Fleas lay tiny white oval shaped eggs. Their larvae are small and pale with bristles covering their worm-like body. They lack eyes, and have mouthparts adapted to chewing. While the adult flea's diet consists solely of blood, the larvae feed on various organic matter, including the feces of mature fleas.

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. Flea bites 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 by carrying Yersinia pestis bacteria. Murine typhus (endemic typhus) fever, and in some cases Hymenolepiasis (tapeworm) can also be transmitted by fleas.