Fleas will bite humans in order to obtain a blood meal. Almost any warm-blooded vertebrate animal can become a target for fleas.
But with that being said, however, most species of fleas have developed to specifically target one type of animal such as a dog or a cat. Those are the primary targets for that species, and they will only attack humans who are in the vicinity of the flea and therefore present themselves as a viable alternative food source.
The fleas attach themselves to the hair or feathers of the animal and lay their eggs usually directly on the host. The areas where these animals sleep and rest are natural places to become infested with fleas and flea eggs.
Although fleas do bites humans, it is not normal for a human to become infested, so to speak, with fleas. Once the flea problem with the animal has been properly addressed, the flea problems with humans will also disappear.
There are a variety of medicines for flea infestations designed specifically for certain species of animals, such as dogs and cats. They are available over-the-counter or through your local veterinarian office.