Disinfest the Environment
Fleas live on their host where they lay eggs which will remain both on the host, and fall off into the host's environment. This means that any thing that the infested person or pet routinely comes into contact with potentially contains dozens of flea eggs. The eggs themselves can remain dormant for nearly a year, and spring to life when a new host's presence is detected so it is important to eliminate all of them.
If a human is infested, launder all of their bed linens, used clothing, hats, steam clean and vacuum the carpets, auto headrests, and any other surface that the person routinely comes into contact with. Carpets are the number one cause of re-infestation so take care when cleaning them. After vacuuming remove the vacuum bag and seal it in plastic to prevent any survivors from getting loose. Washing everything else in soap and water will kill any remaining fleas.
If a pet is infested, quarantine them into one area to keep them from distributing more eggs into the environment. Steam clean your carpets and then vacuum well, sealing the used vacuum bag in plastic. Launder any bedding that the animal uses and drench outdoor resting areas with soap and water to wash away and kill any stragglers. Once again, be very thorough cleaning your carpets!