If allowed to get out of control, they can be very dangerous, robbing your cats of strength and nutrients (they literally starve to death), affecting their immune systems, cause nasty diarrhea and just generally making them unhappy cats. Also, there is a slight chance you could somehow pick up and ingest a tapeworm egg from cleaning the litter box, or petting your cat. You can get a general dewormer from any pet store if you want to spare the expense of a vet visit and fecal exam, and they are usually effective on even a cat that has a bad infestation. If the cat doesn't show any improvement after a week after last treatment (it is usually a 2-pill treatment a few days apart), then it is time to go to the vet. And yes, the dewormers from pet stores (NOT Walmarts) address tapeworms. Hell, I should know, with 3 outdoor rodent-catching cats.