Most fleas are no more than a nuisance to humans.
"They generally cause a lot of itching and scratching," says Marcia Larkins of the Food and Drug Administration's Center for Veterinary Medicine. "They may also cause some discomfort due to possible allergic flea bite dermatitis."
One type, the oriental rat flea, transmits plague to humans in Asia, Africa, and South America. Both male and female fleas can transmit the infection.
Some pets are extremely allergic to flea bites. In these pets, fleas may cause a rash, inflammation, and hair loss. In response, cats may compulsively overgroom, according to the FDA.
Fleas are hard to spot — look for their black, poppyseed-like excrement.
A recent study found that simple vacuuming is an effective way to rid your home of fleas.