Utah State University Extension
Fleas transmit disease and parasitic worms, and cause allergic dermatitis to humans and animals. There are about 2,000 different flea species in the world and at least 325 species in North America, all belonging to the order Siphonaptera. Fleas are not likely to be confused with other insects because adults are flattened dorsal-ventrally, or side-to-side.
Hodgson, Erin W.; Karren, Jay B.; and Roe, Alan H., "Fleas" (2008). All Current Publications. Paper 904.