Flea

Champion jumpers

Fleas can jump up to 150 times their own body length — equivalent to a human jumping 250 meters. The jumping mechanism uses resilin, a rubber-like elastic protein:

1. Muscles slowly compress the resilin pad in the leg.
2. A catch mechanism holds the energy stored.
3. When released, the resilin springs back in less than a millisecond, flinging the flea into the air.

Fleas accelerate at about 140 g during the launch — far higher than any human-built device. The rapid release is what produces the extreme jumping.

A complete metamorphosis

Despite the impressive adult, only a fraction of a flea infestation is the visible adult fleas:

• Eggs (50%) — laid on the host, fall off into the environment.
• Larvae (35%) — small, worm-like, feed on adult flea feces and organic debris in carpets and pet bedding.
• Pupae (10%) — develop in cocoons; can remain dormant for months.
• Adults (5%) — what you see jumping.

This is why pet flea treatment requires both treating the animal AND treating the environment — eliminating just the adults leaves the other 95% to repopulate.

A medieval disaster

The Yersinia pestis bacterium — the cause of bubonic plague — is transmitted to humans primarily by rat fleas (Xenopsylla cheopis). The medieval Black Death (1346–1353) killed an estimated 75–200 million people across Europe, Asia, and North Africa — perhaps 30–60% of Europe’s population.

The plague was passed from infected rats to fleas to humans. Rat populations collapsed; fleas sought new hosts; humans became the alternative hosts; the plague spread through urban populations with horrifying speed.

Sporadic plague outbreaks still occur — primarily in the western United States, Madagascar, and parts of Asia — but modern antibiotics make plague treatable when caught early.

Cat fleas everywhere

The cat flea (Ctenocephalides felis) is the most common pest flea on dogs, cats, humans, and many other mammals. Despite the name, cat fleas opportunistically bite anything warm-blooded. Modern flea-control medications (oral and topical) have made flea infestations far rarer in pet households than they were in the 20th century.