Why do dogs get worms?

Intestinal worms are parasites, which are caused by the dog being exposed (usually by eating something unpleasant) to the immature forms of the worms. Dogs can also get some worms from insects biting them, such as fleas carrying tapeworms and mosquitoes carrying heartworms. Ticks are also a known reservoir of internal parasites for dogs, along with several nasty bacterial infections like Lyme disease.