Behold, the flea—one of nature's most unsavory characters. Fleas survive essentially everywhere, infesting wildlife, pets, and yes—even humans. These flattened insects can slip between hairs in search of a juicy vein from which to feed. They can also survive months without eating, but beware! If fleas have proliferated but had no food for a few weeks, they will be hungry, and the first warm creature to come near will be inundated with hundreds of fleas in a feeding frenzy! They can hide out in such unlikely places as a pair of fuzzy slippers, later jumping onto and biting the victim's legs.
Fleas spread plague so efficiently because the bacteria clog their guts, making them unable to digest the blood they drink. Thus, they are especially hungry, so they jump from host to host, trying to feed. When they try to suck blood, their stomachs are too full, and they end up regurgitating some of the disease-causing bacteria into a new victim.
Thankfully, we've learned to control fleas and disease to a large
extent, but as always, they continually adapt to our defenses. 
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
Contributor: Kodi R. Jeffery, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
A sketch of one of the common fleas. Adapted from Ross, 1956.
Medvedev S.G. 1977. Host-parasite relations in fleas (Siphonaptera). I. Entomological Review, 77 (2):318-337.
Medvedev S.G. 1977. Host-parasite relations in fleas (Siphonaptera). II. Entomological Review, 77 (4):511-521.
Ross, H. H. 1956. A textbook of entomology. 2nd ed., John Wiley & Sons, New York, 519.
Zoological Institute, St. Petersburg. A broad source of primarily technical information on fleas.