A computer software salesperson travels extensively in Asia and parts of Europe.Recently when he returned home from a month of business activities in Southeast Asia,he quickly developed severe diarrhea and intestinal discomfort that worsened over aperiod of several weeks. His family physician suspected either a bacterial or a parasiticinfection, but leaned toward a diagnosis of a parasitic infection because the patient’stemperature was not extremely abnormal. A stool specimen sent to the laboratory ofthe local hospital was negative for the presence of bacterial pathogens but revealed alarge protozoan which contained cilia around the entire periphery of each organism.What was the most likely diagnosis for this parasite, and is it nonpathogenic?