Oysters, like other shellfish, feed by passing large quantities of water through their digestive systems. So if they live in contaminated water, they can pick up bacteria and viruses.

Cooking the shellfish kills these, which is why many people eat only cooked oysters.

If you like to eat oysters raw, you should know these things about protecting yourself:

* Seek certification. Buy only oysters that carry certification. Certified oysters have labels on their shipping containers that tell, bay by bay, which body of water they came from and who shipped them. This certification can't guarantee non-contamination, but it does show the oysters came from approved waters. Oysters with no certification may have been harvested illegally from polluted waters.

If you are concerned, you can ask shop owners or restaurant staff to show you the certification. They are required to keep a record of it for 90 days. If you intend to eat oysters or any fish raw, it's good to deal with a high-quality market that will help you do it correctly.

* Keep 'em alive. Keep live oysters well chilled. Store in the refrigerator under well-ventilated conditions, not in an airtight bag or under water, where they can suffocate. If a shellfish is open when you want to eat it, tap it sharply on a plate or counter. If it doesn't draw closed, throw it away.