Yep, you can have an "infestation" while only seeing a few visible fleas. It is thought that when fleas infest a house the visible adult fleas only make up 5% of the flea population. The rest are eggs, larvae, and pupae. So for every 5 adult fleas that might be on your pet there might be almost 500 more in the house in various stages of development! And since each adult flea can lay more than 500 eggs in its lifetime... you can see how easy it is for thing to get out of hand fast.
I have taken care of flea problems without spraying the entire house. Yuck! I used a good topical flea treatment (Revolution) year round and vacuumed religiously for a month.
It does stink to have an infestation. I was dealing with one three months ago: apparently the cat I was taking care of had fleas for months (I thought the owners treated the cat, but they hadn't and he lived with me for three months), and then my pup came home with fleas from the shelter, too. We treated both animals with Revolution, vacuumed all carpet areas every day, and vacuumed off whatever furniture the animals had been on daily (the cat likes my enclosed sun porch furniture).
Revolution and the like takes care of the active fleas, and also makes their eggs sterile. But then, of course, you have flea eggs, larvae, and pupae still in the house from before you treated, and you just can't get them all with the vacuum or chemicals. It can take up to 8 months for a flea to make the transformation from egg to breeding adult. And the pupae can stay dormant for a LONG time in your carpet and furniture. Chemical insect treatments will not kill them, so all that spraying your vet recommends will be useless.
The pupae are triggered out of their dormancy by heat and vibration (like a pet laying down on the carpet, or a vacuum cleaner running). So unless you continue to treat your pet for fleas through the winter you may still get fresh fleas on your pet from inside the house that were laid before you treat with Revolution, Frontline, or whatever. And of course they can still pick them up from outside.
I have heard of people having good results with DE, but honestly for me that is too much work, and I really prefer to just do a topical flea/tick/heartworm/earmite treatment year round.
If your dog has had fleas it might be wise to do a round of deworming for tapeworms. They are spread by fleas. My sister's dog was just diagnosed with tapeworms from fleas. Yuck!