The cat flea is a small sucking insect of the order Siphonaptera. Adults range from 1–2 mm long and are usually a reddish-brown colour, although this can vary. The cat flea, and all other fleas, are compressed laterally, resulting in an extremely thin insect that can be quite hard to find in an animal's coat.
The cat flea's primary host is the domestic cat, but it is also the primary flea infesting dogs in most of the world. The cat flea can also maintain its life cycle on other carnivores and on omnivores.
Humans can be bitten, though a long-term population of cat fleas cannot be sustained and infest people. However, if the female flea is allowed to feed for 12 consecutive hours on a human, it can lay viable eggs.