Dogs, especially puppies, are commonly infested with parasites such as worms. Many times, the infestation is not noticed until it has grown out of control and the animal becomes sick. Your pet can have worms even when fecal samples are tested and come back negative for parasites. Most parasites can be controlled by medication, although some cannot be completely eliminated because of dormancy of eggs and larvae. The best way to control a parasite infestation in your dog is to prevent it from occurring in the first place. Regular and diligent preventative treatment and exams will help ensure no infestation in your dog.