flēah

Old English flēah comes from Proto-Germanic *flauhaz, related to the verb 'to flee' (Old English flēon). A flea is, etymologically, the thing that flees — that leaps away when you try to catch it. The connection between flea and flee is one of the more transparent etymologies in English, though most speakers do not notice it. German Floh, Dutch vlo, and Swedish loppa all come from the same root. The insect was named for its escape, not its bite.

Fleas killed more Europeans than any other animal in history. The bacterium Yersinia pestis, which causes bubonic plague, is transmitted by the oriental rat flea (Xenopsylla cheopis). The Black Death of 1347–1351 killed an estimated 25 to 50 million people in Europe — between one-third and one-half of the continent's population. The flea was the vector. The rat was the host. The bacterium was the weapon. None of them knew it. The germ theory of disease would not emerge for another five hundred years.

Flea markets have nothing to do with actual fleas, or maybe they do — the etymology is disputed. The French marché aux puces (market of fleas) first appeared in reference to a market in Saint-Ouen, Paris, in the 1880s. One explanation is that the secondhand goods sold there were so old they might harbor fleas. Another is that the market's crowded, jumping chaos resembled a flea's behavior. The Paris market still operates. The name spread to every language in Europe.

The flea's jump is powered by a protein called resilin, the most efficient elastic material known to biology. Resilin stores and releases energy with 97 percent efficiency — better than any rubber or spring humans have manufactured. The mechanism was not fully understood until 2011, when researchers at Cambridge demonstrated that fleas use their toes, not their knees, as the launch platform. The thing that flees does so with biomechanical perfection.