Preventing parasites in your pet is important for their health — and yours. There are a number of parasites that infect dogs and cats internally and externally, and they can infect you as well. Common external parasites include fleas, ticks, and mites. The most common internal parasites are heartworms and intestinal parasites such as Giardia and worms (e.g., roundworms, hookworms, whipworms, and tapeworms). In most cases, intestinal parasites cannot be seen by the naked eye. Laboratory testing is required for diagnosis and treatment.