We also have dogs and cats present with fleas. They may be in areas where other carriers have been, in a house or other infested environment (barns, dog house, household furniture, grooming facilities) or direct contact with infected pets (dog park, doggie daycare etc.) A large percentage of the local wildlife are potential carriers and your pets exposure rate is probably far higher than you may think. Animals that tend to hunt rodents, are also at increased risk of ingesting fleas. Fleas are also the carrier for tapeworms (intestinal parasite).
Treating your pet as soon as possible after finding signs is extremely important in preventing an infestation. They can quickly lay eggs in your furniture, carpeting, car seats and anywhere your pet goes, continuing the cycle. Humans will not become infested but can sustain uncomfortable bites.
There are simple topical and oral preventatives available for cats and dogs. Ask your veterinarian what the best option is for your pets.