Coined by German botanist Heinrich Anton de Bary in 1879, symbiosis is defined by biologists as: "life together." In symbiotic relationships two organisms live in close association with each other. These relationships are categorized into three types depending on the role each member plays in the association: mutualism, commensalism and parasitism.
Mutualism is a symbiotic relationship where both organisms derive benefits from the other. This includes the microorganisms living on the skin and in the digestive tracts of humans. There are more bacterial cells living on and within the human body that there are actual cells in the human body. Mutualistic relationships are either obligate, where the relationship is essential to at least one of the organisms or facultative, beneficial relationships, but not essential for survival.
Commensalism generally means that both organisms receive neither benefits nor harm from the other. However, it is not as clearly defined a category because, through further study, benefits and or harm to one of the species involved is often discovered.
Parasitism occurs when an organism feeds or is sheltered by another organism without any benefit to the host. Often the parasitic species will develop into a pathogen, causing harm to its host organism (Hartnett 2010).