Endoparasites are one example, most prominently tapeworms. Cats are rampant with carrying tapeworms, which are spread by fleas. The cat has to eat a flea in the process of scratching with their mouth predominantly. The fleas carry tapeworm eggs, and that's how tapeworms become infested in the cat.

The life cycle for ticks goes from adult to egg, larva, nymph, and new adult.

The life cycle for fleas goes from mama flea to eggs, larva, pupa, and adult flea. This process takes about a week.

Yes, because they're all going to go to whatever is the most available food source. If you bring in a stray cat with fleas and you have other pets that you haven't had on flea prevention, it won't take more than about three days before everybody is infested. They can also affect people, but humans don't have a lot of hair, so we don't have a whole lot of ability for them to create skyscrapers on us.

On a cat, fleas will live anywhere, but you're most commonly going to see them around the neck, at the base of the tail, and on the belly. They like the areas that are not easily exposed to biting and scratching for obvious reasons.