Yeah, unfortunately all it takes is one flea, and fleas can live in carpets for a long time. They will on occasion also hide on clothes and shoes of people they come in contact with if desperate. My cat came home flea free, to an apartment that was supposedly flea free (or so we were told, they apparently bomb for fleas before moving a new tenant in if the previous occupant had a pet), but about 2 months in he had the worst flea infestation ever. The way we figured, the super was probably going from apartment to apartment, and one flea hitched a ride on his clothes and right into our apartment.
So within a month my (then) kitten had to go through a flea infestation (so he got pilled and put on a drop treatment routine), and as soon as that stopped, I found tapeworms on my couch (yuck! I steam cleaned the whole thing, but have since also gotten rid of it too), so that he had to get pills for that too: 1 immediately, and one 10 days later. At the end of that month he was scared of the carrier and it's taken months for him not to associate car rides with going to the vet.
Thankfully ever since he was put on a preventive he has been flea free, so that took care of the whole thing. =P