Before you self-diagnose your symptoms as due to a parasite, it’s important to understand what a parasite is. “A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host,” according to the CDC. There are three classes of parasites: Protozoa, which are tiny, one-celled organisms that typically live in the intestines, blood, or tissue; helminths, which are parasitic worms such as tapeworms, roundworms, and thorny-headed worms; and ectoparasites, which are ticks, fleas, lice, and mites that attach to or burrow into the skin. Parasites can cause disease and even death, but fortunately, if caught early, the infections can usually be treated with medication. (And sometimes they are just annoying and icky, but not necessarily life-threatening—like head lice.) Here are some silent signs and parasite symptoms that could mean you are hosting one of these tiny invaders, including the parasite that millions of people in the United States have and don’t know it.