We use the flea repellant that you put on the base, of their neck. Works well. Sometimes it can cause a reaction, on some animals. Straay dogs, and perhaps cats, can cause big problems. They will find a patch of damp dirt and lay and wallow in it and a bunch of fleas will fall off or leave or something. I've watched foxes go into freshly sprayed fields and just turned fields and lay out as long as they can stretch. Never understood it until Dad told me they got rid, of fleas or some fleas, that way. The fields may have all had poisons applied to them.
Handle one of these critters,loaded with fleas and you are fresh meat.
Go near an infested building or home and you'll take a bunch with you, many times. I guess it depends on the number of them.