Fleas are parasites that feed on the blood of any warm-blooded body. The most common species of flea is the cat flea, which often feeds on cats, dogs, and us. Fleas will usually remain on the host at all times but have been known to start infestations inside the carpet fibers, furniture. and even the yard. Normally, fleas are going to torment your pets but as the infestation grows they will start biting you.