The term "shellfish" in this context is limited to oysters, clams and mussels. Most shellfish thrive in estuaries with mixtures of fresh and saltwater. Shellfish are filter feeders, which means that they get food and oxygen by pumping large quantities of water across their gills. During feeding, shellfish take in bacteria, viruses and chemical contaminants, and can concentrate these impurities in their digestive systems and tissues over 100 times the levels in the water. Because oysters, clams and mussels are often eaten raw and partially cooked, shellfish harvested from polluted areas are a health hazard if consumed. Diseases resulting from consumption of shellfish harvested from polluted waters include typhoid, hepatitis and salmonellosis.
