Symbiosis is a close, long-lasting relationship shared by two or more different species. There are 3 major categories of symbiotic relationships. Parasitism is a symbiotic relationship in which one organism benefits while the other is harmed. For example, tapeworms are parasitic to humans because they consume nutrients from the human host, depriving the host of these nutrients. Mutualism is a symbiotic relationship in which both organisms benefit from the relationship. Commensalism is a symbiotic relationship in which one organism benefits while the other is neither harmed nor helped.