Most bloodsuckers flee after stealing a meal, but not fleas. Rather than commuting to and from a host like mosquitoes or bedbugs, fleas often just hang out in its fur. They're well-suited to this lifestyle, thanks to thin bodies that help them slink through fur, hard shells that make them difficult to crush, and spring-loaded legs that let them jump 7 inches high and 13 inches across (see the video below for more on flea acrobatics). In human terms, that would be like leaping 250 feet high and 450 feet across.
Different flea species target specific hosts — there's a dog flea, cat flea, rat flea and even human flea — although they're not averse to mixing it up, as many pet owners can attest. That's how rat fleas spread bubonic plague around Europe during the Middle Ages, and still do in some parts of the world.