How can one dog attract tapeworms from another dog? How to prevent it?

We have a 3 year old dog right now. We just bought a puppy and she is infested with tapeworms. How can our 3 year old attract tapeworms from the puppy? (ex: drinking from the same water dish, sniffing each others butts, eating from the same food dish, etc?) How can we prevent our 3 year old from becoming infested. We bought a pill for the puppy to kill the tapeworms and we are taking her to the vet tomorrow.

The way a dog contracts tapeworms is by eating a flea. If your 3 year old dog has fleas (even just one), and it gets on the puppy, the puppy bites to scratch itself, then that flea can be swallowed. Drinking/eating from the same bowl and sniffing butts doesn't transmit tapeworms.Not all dogs with fleas have tapeworm. The flea would have to be carrying the larval form of the tapeworm, AND get eaten, in order for a dog to have tapeworms.Most dogs that have tapeworms have fleas, though.

Dogs don't get tapeworm from other dogs, they get them from ingesting fleas. All dogs with fleas have tapeworm.