The Case Record contributed by Hohmann et al. (Aug. 14 issue),1 regarding a man with ulcerative colitis in whom bloody diarrhea developed soon after fecal microbiota transplantation, highlights the difficulty in distinguishing cytomegalovirus (CMV) as a direct cause of disease from its being an “innocent bystander” in patients with CMV infection and active ulcerative colitis.

However, the authors also reported an apparent case of diarrheal disease caused by Blastocystis hominis transmitted by means of FMT without describing the complex debate as to whether this organism is a true enteropathogen.

In developed countries, blastocystis is detectable in more than 50% of healthy persons with the use of polymerase-chain-reaction–based assays.2 No difference in the prevalence of blastocystis exists between hosts with gastrointestinal symptoms and those without such symptoms,3 and symptoms that are attributed to blastocystis infection may not improve even after the elimination of the organism.4 The organism has been shown to be capable of both long-term colonization2 and spontaneous disappearance without intervention.4,5 Physicians offering FMT should carefully monitor FMT recipients for infectious complications but must be aware that presence is not the same as causation.