I can't help on the garlic issue, but if you are looking for a natural flea killer try DE...
"Diatomaceous earth (pronounced die-ah-toe-may-shus) is a finely ground fossilized diatom, a one-celled algae with shells. Diatomaceous earth can be found in vast deposits originating from ancient oceans over much of the world. Much of what is solid in this country is mined in the southwestern United States. When this material is finely ground, the microscopically sharp edges of the particles pierce the protective coating of insects so that they dry out in a few hours and die. In addition to its use for eradicating fleas and ticks, it is used by gardeners as a non-toxic dust to control insects on plants, as a natural means of fumigating grains and seeds for long term storage, and to aid intestinal tone and prevent worms in animals when added to a pet’s food on a daily basis. The U.S.D.A. has approved up to 2% by weight of ration as animal feed supplement to control internal parasites, including worms transmitted by fleas. The diatomaceous earth passes harmlessly through the digestive system. Use one teaspoon daily for cats, small dogs, and puppies and 1 tablespoon for dogs over 55 pounds.
The effectiveness of using diatomaceous earth for external parasites is related to the thoroughness of application. 1 cup will deflea a dog. Put it in a salt shaker or similar container and shake it on the animals extremities, and combing it through to get it down to the skin. Be particularly thorough around the ears, between the legs and around the tail. Repeat this process in 7 days and then in another 7 days after that. Since the diatomaceous earth particles can irritate the eyes and the respiratory system, keep the dust of the animal’s eyes, nostrils, and mouth. These precautions are equally important for you. A dust mask may be advisable when working with a large quantity. Refrain from applying diatomaceous earth on windy days.
Never use filter type diatomaceous commonly used in swimming pool maintenance. This type is treated until it no longer resembles the state in which it left the mine. After being air dried, it is treated with soda ash and placed in a kiln. At heats of 2000 degrees, some of the residue is burned off and the primary ingredient, smorphous silica, changes physically and chemically into needles of glass with a tough, ceramic-like coating.
Herbal flea collars help prevent fleas from hopping on for a ride. Unlike dimethydichlorovinyl phosphate (DDVP), a spin-off from nerve gas warfare research that is found in chemical flea collars, herbal flea collars take advantage of the benefits of essential oils. Two types are available – a plastic collar impregnated with herbal oils and a rechargeable cotton variety. Herbal flea collars work best in areas of low infestation. Although not something I recommend, they are available."
http://www.carefreepet.com/natural_flea_control.html
I have never used the stuff, but have heard great things about it on other forums regarding the treatment of fleas.
Might be something you can look into more on your own.