If left untreated, parasites can cause diarrhea, vomiting, anemia, malnutrition, and even - in rare, extreme cases - death. Parasites are common, especially in puppies and kittens, and can be eliminated with medication. Some parasites can be transmitted to humans, usually through accidental ingestion of microscopic amounts of stool - though hookworms can enter the body through the skin, and are therefore a risk if your pet goes to the bathroom in the yard and you then walk there barefoot.