Why Do Fleas Bite Humans?

Fleas bite humans to obtain blood, their source of energy and nutrients, states the Illinois Department of Public Health. The human flea prefers humans and pigs, but humans are also bitten by other species of flea such as cat fleas and rat fleas. While different species of flea prefer to prey on different species, they can be easily transferred to other species in close proximity. In the United States, cat fleas bite humans the most often.