There are more than 2,000 known species of fleas ! Just one is the most typically dealt with in the U.S. and whether it's on your cat, dog, ferret, hamster, squirrel or skunk - is called the "cat flea". There is a species of flea called the "dog flea", but they are rarely found in the U.S.. The "Sticktight flea" can become a pet problem if they frequent barnyards with poultry. For now though, let's just assume this is the common, ordinary, everyday little pest:
Fleas need a blood meal to live and procreate. So while they may be carried on or in cardboard boxes, or even on clothing, shoes, etc. - they can't live and thrive without getting a blood meal.