Fleas are able to jump to a distance of about 4 inches. This distance does not seem all that long to humans or most other animals. However, that distance is actually about 100 times the flea's height.
The ability to jump that far and that high is the reason fleas are able to jump onto cats and dogs. When comparing the size of a flea to that of a house pet, the pet would appear to be too tall to reach. However, the flea can use its ability to jump high and far to hitch a ride on the creatures it prefers.