Fleas rarely jump from one pet to another, as is often thought. Instead cats and dogs pick them up from infested environments. This could be your garden, the local park, a friend's house – any place where an animal that has fleas, such as a rabbit, hedgehog, fox or another cat or dog, may be found. Infested animals leave flea eggs behind wherever they go. New fleas hatch from these infested environments once they sense the warmth, carbon dioxide and vibration an animal such as your pet creates, and jump onto them.