Many organisms live on or in other animals, causing no problems, even producing benefits for their "host". Bacteria on the skin that help support the immune system are an example of a productive, mutually beneficial relationship. Parasites are organisms that live on or in animals, but offer no benefits and may be detrimental, drawing their nourishment from their "host". Parasites vary in size from microscopic single-celled organisms through just visible skin mites to intestinal tapeworms metres long. Parasites are usually divided into two categories, those on or in the skin are called external parasites or ectoparasites while those inside the body, in the intestines, heart, lungs liver, kidneys and brain are internal parasites or endoparasites.