Blastocystis hominis, a long-misunderstood intestinal parasite.

For over 50 years, Blastocystis hominis has been held to be a harmless intestinal yeast-probably frequent in stool samples from man and other primates, but usually ignored except as a possible source of confusion with Entamoeba histolytica. More recently, its status as a protozoan parasite has been accepted, and it is now increasingly recognized as an agent of intestinal disease - usually self-limiting but occasionally fatal in monkeys. Here, Charles Zierdt reviews the status o f this intriguing protozoan, drawing attention to its unusual biochemistry.