I stole this from here:
http://www.caberfeidh.com/Fleas.htm
Fleas, like all parasites, prey on the weak, sick, and malnourished. An animal infested with fleas is an unhealthy animal, and while the fleas certainly worsen this ill health, they do not cause it. Poor-quality, inappropriate diets cause parasite infestations, as do unsanitary conditions and all forms of stress. By knocking out the flea, whether with insecticides, nematodes, or hormones, you have done nothing to solve the root problem of ill health.
The first and most essential step is to improve your animals' health and vitality. The single best way to do this is to feed them a diet based on nature, rather than a highly processed commercial food. This single step is 99 percent of the battle, and yet is the one most people don't want to consider when looking for an alternative to pesticides.
Some good ideas can be found in the following links:
http://www.stretcher.com/stories/980716c.cfm
http://www.extendedyears.com/forum/fleamess/4285.html