Some kinds of tapeworm result from a dog eating a dead rabbit or rodent, but the most common one has part of her life-cycle in fleas. When the dog nibbles and grooms herself, she picks up the infestation from the fleas. So having your dog appropriately wormed is not enough. You must also eliminate the fleas to prevent reinfestation. You may even have to fumigate your house to get rid of them.
Some dogs develop severe allergic reactions from fleas.
If you let your dog roam free and unsupervised, the dog is much more likely to pick up fleas, worms, and more serious infectious diseases from other dogs. Your dog may also get injured in fights with other dogs or hit by a car. For health's sake, keep your dog indoors or in a clean pen or yard outdoors, and take your dog out regularly on the leash for exercise. Dogs, like people, get ill periodically. Has a dry, hot nose, red eyes, and seeks a quiet, cool place.