You don't have to have a pet to get fleas in your lawn or home, fleas can survive just as well on human blood as they would on your pets. Wildlife carrying fleas will pass through your yard or rest under your deck and will drop dozens of fleas onto your property.
Great! You've bombed the house and went on that two week vacation you've been dreaming about expecting every flea in your house to have starved at this point, and you would be correct...if it weren't for the few dozen eggs buried in the carpet. In ideal conditions flea eggs can hatch as early as 2 days and typically up to 2 weeks; however when certain conditions are met flea eggs will become dormant and can sustain suspended up to 6 months.
Treatments will depend on the level of infestation, whether your pet has been treated (if applicable), and/or removing the wildlife that are reintroducing fleas into your yard. One-time services as well as regular maintenance plans are available.