During the Middle Ages, tens of millions of people died of the plague, or Black Death, as it spread across Asia and Europe. Cities were particularly hard hit. London lost 20% of its population to the plague in just two years during the mid-1600s. It wasn't until the dawn of the 20th century, however, that we identified the cause of the plague – a bacterium called Yersinia pestis. What does this have to do with fleas? Fleas carry the plague bacteria and transmit it to humans. An outbreak of the plague often kills a large number of rodents, particularly rats, and those bloodthirsty, plague-infected fleas are forced to find a new food source – humans. And the plague isn't a disease of the past, either. We're fortunate to live in an age when antibiotics and good sanitation practices keep plague deaths to a minimum.

Without intervention, a few fleas on Fido can quickly become a maddening flea infestation that feels impossible to defeat. That's because fleas, like bed bugs and other bloodsucking pests, will multiply quickly once they've found a good host animal. A single adult flea can lay 50 eggs per day if it's well fed on Fido's blood, and in its short lifespan can produce 2,000 eggs.

Remarkably, scientists estimate there are over 2,500 species of fleas on the planet. Within the lower 48 U.S. states, flea species number approximately 325. But when fleas infest a human habitation, they're almost always cat fleas, Ctenocephalides felis. Don't blame kitties for this annoyance, though, because despite their common name, cat fleas are just as likely to feed on dogs as they are on cats. Dog fleas (Ctenocephalides canis) can also be a pest problem but are mainly found on dogs that spend all or most of their time outdoors.

Fleas don't fly, and they would never be able to catch your dog in a foot race (despite having six legs to Fido's four). So how are these tiny insects able to get around? Fleas are amazingly adept at flinging themselves into the air. Cat fleas, our most common flea pest, can propel themselves a full 12 inches forward or upward. That's a jumping distance equal to roughly 150 times its own height. Some sources compare this to a human landing a long jump of nearly 1,000 feet.