This cleared out a flea infestation in our dog. We use it preventatively because ticks, as well.
Fleas tend to linger because their eggs shed all over the place. As I recall, frontline had some double action going on by both killing fleas and causing their eggs to hatch into nymphs that never evolve into breeding maturity.
Fleas don’t really like biting humans, so any occurrence is a one+off.