“Lie down with dogs, get up with fleas.” That proverbial warning is, if anything, too narrow. These wingless insects are far from picky and can inhabit a variety of warm-blooded hosts, including humans. The flea’s host-hopping—made possible by long legs that can jump up to 12 inches (30 centimeters)—helps spread plague, typhus, and other ailments to people and animals. AVERAGE SIZE: 0.4 inch (9 millimeters).