The key to successful flea treatment is remembering that only the adult fleas live on your cat. These adults only account for around 5 percent of the total flea population; the other 95 percent live in your pet's environment as eggs, larvae and pupae.
The adult flea feeds on blood by biting its host. It then lays eggs which fall to the ground. When the environment is right (preferably humid and warm) these eggs hatch to release larvae. The larvae feed on bits of debris and adult flea faeces on the ground. Larvae dislike strong light and burrow deep into carpets (or grass etc outside). In the winter in the UK many flea larvae will die outside, but they will survive well in centrally-heated homes. The larvae form pupae and emerge as adult fleas around a week later, stimulated by the presence of a potential host (eg a passing dog or cat). Pupae are resistant to insecticides and can survive for several months if there is no host around, so adult fleas can appear even months after insecticides have been used. If conditions are right, the whole cycle from adult flea to egg to larva to pupa to adult flea can take just 2-3 weeks.
There are a variety of different products available for preventing and treating flea infestations, including spot-on preparations, sprays and injections. Many are only effective against the adult flea so must be used regularly to prevent a build-up of fleas. Where there is existing heavy infestation it is often advisable to treat all your animals (both dogs and cats as they can both be affected) and also use a spray to treat your house.
If there is a heavy infestation of fleas in a young animal, anaemia can result due to the amount of blood the fleas are taking to feed themselves. Furthermore, fleas can carry disease and spread infection to dogs and cats, include the common tapeworm.
Finally, some cats are allergic to fleas and even a small number of bites can result in redness, itchiness, bald patches and scabs over large areas of the body. This is called 'flea allergy dermatitis' or 'FAD' and strict flea control is needed to prevent skin irritation. If your cat is showing any signs of skin disease you should consult your vet.