Symbiosis is a biological relationship in which two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms. When both partners benefit, this variety of symbiosis is known as mutualism. The name for a situation in which only one of the partners benefits is far more well known. Such an arrangement is known as parasitism, and a parasite is an organism that obtains nourishment or other life support from a host, usually without killing it. By their very nature, parasites are never beneficial, and sometimes they can be downright deadly. In addition to the extremes of mutualism and parasitism, there is a third variety of symbiosis, called commensalism. As with parasitism, in a relationship characterized by commensalism only one of the two organisms or species derives benefit, but in this case it manages to do so without causing harm to the host.