Pretty much all households with pets probably have fleas unless they're using very strict flea control. Reasoning: Even if your cats don't go outside, YOU and (if you have dogs, they) do. All it takes is brushing up against a bush that the local stray cat (aka, "FleaBus") was in a couple hours before. A couple fleas jump on your pants, then jump off when you get home. Human blood does not nourish the flea sufficiently for it to reproduce, but dog and cat blood does.
Now that flea does its usual thing (jump on dog or cat, take blood meals pretty regularly for the next couple weeks, lay tons of eggs, and die). Those eggs are now waiting for the ideal environmental conditions to hatch out (or if they already have, for the larvae to pupate). That includes temperature, humidity, vibrations, carbon dioxide levels - things that tell it that there are mammals around. Most human households are kept (even in "the non flea season" of winter) at temperatures that make fleas happy. Flea eggs and pupae can wait for YEARS for the right conditions.
Say you only have one or two fleas do this a month for several years. Now you've got thousands or millions of fleas just waiting. And maybe now is the time.
Just because you don't have a "flea problem" doesn't mean there are no fleas. By the time you see fleas on your pet, there's dozens more that you don't see on the pet at the same time. Even in really short-haired dogs, fleas can hide. Their body shape is designed for it.
Start all dogs and cats on strict flea control (an adult-killing product, such as Frontline, Advantage, Revolution, Comfortis (dogs only), depending your vet's preference). Frontline and Advantage must be applied every 3 weeks.
Vacuum thoroughly. Every nook and cranny and corner. Daily. Empty the bag outside, daily. You're doing two things: getting rid of some of the sheer number of flea eggs/larvae/pupae. And the vibrations encourage the fleas to move on in their life cycle. They become adult fleas, come visit your pets, and die. You WANT your pets to have access to all possibly contaminated areas - because they are the method of killing the fleas, how that you've got flea-killing products on/in them.
Wash every cloth-related object that can be (pillows, dog/cat beds, comforters). Wash the covers of everything you can't wash the whole thing, and preferably, let what can be, sit in the sun for a day or two. Destroy and replace anything that can't that you can reasonably get rid of. You've probably got flea eggs and larvae in mattresses and couches, which are harder to clean.
Flea bombing is up to you. You shouldn't *need* it, but it will make things go faster.
Avoid most OTC flea products - things like Hartz and Sargent's and whatnot typically have very toxic chemicals in them, which can be dangerous to dogs and cats, and they don't always work. Flea collars only work where the collar is (around the neck). Flea shampoos only work as long as the shampoo is on the animal (minimal residual effect). Frontline and Advantage are available OTC. If you get Advantix for any dogs you may have, keep them away from the cats (it's got a toxic to cats chemical in it).
Keep an eye out for tapeworms - fleas carry them, and animals are infested by chewing/biting at the flea and accidentally swallowing it. It's easy to treat, and once you get rid of all fleas, you don't have to worry about them in the future.
Don't blame the vet - yes, that's a possible place fleas could have come from, but so is your backyard, the dog park, or just the fact that they may have been brewing for a while. Maybe your local weather changed recently, encouraging fleas to come out?
And regardless of how you end up controlling the fleas... it takes patience, sometimes months worth. Keep your pets on a good flea control regimen. Once you've finished with the current infestation, ask your vet about Program - it's flea birth control. The flea does its usual thing, but all the eggs are sterile, making it less likely that you will have this problem in the future. IMO, it's best for indoor-only animals, but even if your cats go outside, it'll help. Program for cats does come in an injectable format (given every 6 months). For dogs, it's combined with Interceptor (for heartworm/intestinal parasites) to make Sentinel as a monthly tablet. Or you can continue to use a monthly adult preventative - especially if your cats go outside, I like Revolution because it does heartworm and intestinal parasites as well as fleas.
Good luck.
