Fleas are parasites.
They inflict annoying bites on a variety of mammalian hosts, including cats, dogs, other domestic animals, and humans. When ingested, they transmit tapeworms, and their bites can cause severe allergic reactions in some of their victims. In mild climates, fleas are not killed by prolonged winter freezes. They thrive in moderate temperatures of 65 to 80 F and in high levels of humidity. These are common weather conditions west of the Cascades in Washington, where, predictably, fleas are also common.