Because they grow in shallow coastal waters, which are likely to be polluted, which shellfish may carry cholera-related bacteria, hepatitis a virus, and the common norwalk virus?

Because they grow in shallow coastal waters, oysters are likely to be polluted. Oyster exists in marine or brackish habitats. There are many different types of oysters, some which are consumed by humans, others that produce pearls and there are the windowpane oysters which are harvested for making decorative objects as their shells are translucent.