Interestingly, several recent studies have found Blastocystis frequently in healthy individuals 7, sometimes at higher prevalence than in those with gastrointestinal disease 8. ...available data suggest that another protist, Dientamoeba fragilis (Fig 1E), has a similarly variable ecological role in the human intestinal ecosystem, occasionally associated with disease 9 but also highly prevalent in healthy individuals 7,8. ...a commensal relationship is more likely for those organisms confined to the lumen of the gut, as exemplified by Escherichia coli, Blastocystis, Dientamoeba, Enteromonas hominis (Fig 1B), and Retortamonas intestinalis (Fig 1C). ...the nature of the host-eukaryote relationship will change depending on the context of the host and the gut ecosystem, so that the same organism may be parasitic in some cases and commensal in others.