They can be viruses, bacteria, fungi, plants, or animals. It turns out most living creatures are parasites of one sort or another. In fact, every single cell in every single organism other than viruses, bacteria, and blue-green algae, contains a former parasite called mitochondria, which basically fuel life. Similarly, plants' photosynthesizing chloroplasts are remnants of ancient parasitic bacteria. While these ancient parasitic relationships seem to have turned out for the best, at least from our point of view, most do not.
Parasites can bring pain, disfiguration, malnutrition, and death.
Some scary parasites enslave their victims, manipulating their behavior and making zombies of the hapless hosts. A barnacle, Sacculina , parasitizes crabs big time. And then it gets worse. Soon a male or two joins the female and they begin producing thousands and thousands of new Sacculina . The poor crab stops growing, stops mating, stops doing anything but eating to feed the parasite and taking care of the Sacculina's multitudinous larvae as if they were its own.