Fleas are tiny parasitic insects that feed on the blood of mammals and birds. You are most likely to encounter them in your home if you have a pet that unknowingly brings them inside. These parasites may be tiny, but they can multiply quickly and create a serious problem for you and your pets. They also multiply quickly, so you need to be able to spot a flea infestation and deal with it quickly if you want to keep it from getting out of control.