The word parasite refers to an organism that lives off of another—in this case your puppy—at the expense of that puppy. Parasites may live on the skin, within the skin, and inside the body such as in your puppy's intestines or bloodstream. They may be bugs or worms you see, fungus that's microscopic, or single-cell organisms like protozoan. These parasites can cause aggravating symptoms at best and deathly illness at worst.