Parasites may be present in food or in water, but cause waterborne illness much more often than foodborne illness in the United States. Parasites range in size, from tiny single-celled organisms to worms visible to the naked eye. They may be transmitted through consumption or accidental ingestion of contaminated water/food, or by putting anything into your mouth that has touched the stool (feces) of an infected person or animal.
The illnesses they can cause range from mild discomfort to debilitating illness and possibly death. The symptoms are typically, watery diarrhea, abdominal cramping, bloating and alternating periods of constipation. Symptoms generally begin a week or so after ingestion of the contaminated item and typically last for several weeks.