Why do fleas live on dogs and cats?

The flea lives on dogs and cats by sucking their blood. Fleas also infest rats, rabbits, squirrels, birds, and nearly all other warm-blooded animals. Even on human beings. During the middle ages, flea-infested rat spread bubonic plague throughout Europe. The flea lived on the diseased rat until the rat died. And then the flea jumped upon a human being, carrying with it the disease germs.

There are actually hundreds of species of fleas. They are parasites. A parasite is a plant or animal that lives with, in or on another living organism.

A flea has a small round head and mouthparts that are adapted to sucking. It has a tiny body, no wings and three pairs of legs.

Did you know a flea is a wonderful jumper? 
It is, in fact, the champion jumper of all creatures. With the help of its long hind legs, it can jump 18 to 20 centimeters into the air and can jump forward at least 30 centimeters!