V. parahaemolyticus grows naturally in coastal waters, and can grow to large numbers when the ocean is warm. Bivalve molluscan shellfish (i.e. oysters, clams, cockles, scallops and mussels) are filter feeders, straining out plankton and bacteria to eat. If the numbers of V. parahaemolyticus are high in the shellfish when harvested, or when shellfish are not kept cold after harvesting, and the shellfish are then undercooked or eaten raw, illness can occur. The illness is not spread from person to person.