Somewhere around Labor Day, most people mentally file "fleas" under summer problems and move on with their lives. Fleas did not get that memo. They don't check a calendar — they check the temperature, and early fall still reads comfortable enough for them to keep going strong.
Fall is flea season's second wind
Veterinary sources, including Tufts University's Cummings School of Veterinary Medicine, note that fleas stay active well into fall and generally don't quit outdoors until the first hard freeze — which for most of the country is still weeks or months away. Until then, it's business as usual outside, and full speed ahead anywhere warm indoors.
Here's the part that actually matters for grooming: this is also the exact stretch when your cat's coat is thickening up for winter. A denser coat is great insulation and lousy flea defense. The extra fur gives fleas more places to hide, and it makes them a lot harder for you to spot with a quick pat-down — which is exactly the kind of thing regular brushing catches early and a casual glance doesn't.
Indoor cats aren't off the hook either
If your cat never sets a paw outside, the seasonal argument doesn't fully apply, and neither does the relief. A single flea that hitches a ride in on a person, a visiting dog, or a pair of shoes finds a warm house in September just as hospitable as one in July. Indoor-only doesn't mean flea-proof. It just means the entry point is a door instead of a backyard.
What to actually do about it
You don't need to panic-buy every product at the pet store. You need eyes on the coat more often than usual for the next stretch. Run your hands through the fur along the back, belly, and base of the tail — the classic flea hangouts — and check for small dark specks that don't brush away clean, or a cat who's suddenly grooming itself like it's a full-time job.
A professional groom does this more thoroughly than a quick check at home, and it catches problems in fur you genuinely can't see once the winter coat comes in all the way. If you've already noticed skin irritation or patches of thinning fur, that's worth a look sooner rather than later.
Fall doesn't end flea season. It just gives fleas better cover.