While on a backpacking trip in Maine, Gemma and her friends…

While on a backpacking trip in Maine, Gemma and her friends obtained their drinking water from several different pristine streams, adding one or two drops of bleach to ensure its safety. Several days later Gemma began experiencing abdominal distress, specifically cramping, fatty diarrhea, and flatulence. When it didn’t clear up within several days, Gemma visited the doctor. Which of the following would be the most likely cause of Gemma’s symptoms?