If your cat has fleas he is also likely to have a tapeworm

The cat flea is part of the tapeworm life-cycle. When a cat ingests a flea while grooming, he might also ingest a tapeworm egg which is inside the flea.

The egg got inside the flea because larval fleas hatching in the area consume tapeworm eggs. These eggs have been deposited on the ground from a segment of tapeworm which has exited a host such as a cat via the anus.

And so you can see how the lives of the two organisms are connected to each other. It probably pays to both de-worm a cat and use a spot-on treatment to de-flea the cat at the same time.