The best flea products are prescription, such as Program® (lufenuron). If your cat already has fleas, then you need to kill them first with a product like Capstar® brand of nitenpyram, Frontline® brand of fipronil or Advantage® brand of imidacloprid. These have residual effects that can also control ticks. A new product, Revolution®, is a topical treatment to prevent external parasites, heartworm and intestinal parasites. Other ideas can be found in Flea Control and Prevention.
In tough cases, you may have to wage all-out war to conquer fleas. This means a comprehensive flea control program, requiring treatment of the pet, the pet's bed, the yard and the house. A variety of sprays, dips, powders, foams and oral products may be recommended.
Ticks are very difficult to control, but a program of tick prevention and meticulously combing and grooming your cat can keep them at bay. See the article How To Remove and Prevent Ticks for more suggestions.
Preventing heartworm disease is easier and much preferred to treating an active heartworm infection. Treatment is easy – just one tablet once a month. See Heartworm Prevention Guidelines for Cats.
Not all parasitic diseases can be prevented but most can be treated. Mites are parasites that can cause serious illness in your cat. For more information, see Ear Mites in Cats, Sarcoptic Mange, Notoedric Mange, Demodicosis and Cheyletiellosis.