Shellfish can often be tainted by certain bacteria, toxic algae, and chemical contaminants. Because some shellfish, particularly mollusks, are often eaten raw, it is imperative that they be purchased from the most reputable dealers. Although the risk of gastrointestinal illness is slight and the infection is usually more uncomfortable than life-threatening, it can be extremely serious for children, the elderly, or people with compromised immune systems.
All legal mollusks are harvested from licensed and inspected areas, and the harvesters are required to tag their catch with their name as well as the area from which the seafood was taken. Most health departments are quite vigilant about this issue, and if any contamination is found, it is usually quickly dealt with.