An organism which lives on another organism whereby the host organism is neither helped (which would be mutualism) nor harmed (which would be parasitism). Examples may include organisms in the human gut flora such as Escherichia coli. There has been some debate as to whether true commensalism exists; for instance, there is some evidence that the gut flora benefits the human by aiding digestion.
Some commensal microorganisms can, under certain conditions, become pathogenic. Examples include Candida albicans and Staphylococcus aureus.