A parasite is an organism that feeds off of another organism. Internal parasites are those that grow by feeding inside the body (worms, for example); external parasites are those that grow by feeding outside the body (fleas, for example). Finding ticks or fleas on your dog is not pleasant — but it's certainly better than not finding them and then realizing you have a flea infestation or a dog who has contracted a tickborne illness! There are other pests that can wreak havoc on your dog, too — microscopic mange and mites and internal parasites in the form of worms. Because these critters are all over the dog's environment, they present some of the greatest ongoing challenges to dog owners. Because of that, there are many products and programs on the market to help control and combat them.