Fleas
Tiny blood-feeding insects that cause itching and skin disease. The most common parasite of pets. Most pets are not "scratching" — they are flea-allergic.
Fleas are far more than a nuisance — they are the leading cause of skin disease in dogs and cats. Most pets that "have a few fleas" are allergic to flea saliva and a single bite triggers days of itching.
Fleas spend most of the life cycle off the pet (in carpets, beds, gardens) as eggs, larvae, and pupae. The pet you see scratching is only the tip — the household has many more. This is why effective flea control requires both the pet (year-round prevention) and the environment (vacuuming, washing bedding, environmental sprays for severe infestations).
Modern monthly chewables (NexGard, Simparica, Bravecto) and topical products (Frontline Plus, Advantage) are highly effective. Many combine flea + tick + heartworm + intestinal worm. Cats need cat-specific products — many dog products are toxic to cats.
Heavy flea infestations in puppies and kittens can cause life-threatening anaemia. Fleas transmit tapeworm (the dog or cat eats an infected flea while grooming).
Year-round prevention is standard in most climates. Lapsing for one summer typically restarts a household infestation that takes 3 months to clear.