There's an old saying that one shouldn't eat oysters in months without an 'R' in them - in other words, the warmer months, from May to August. I was always puzzled by this and thought it to be hemisphere-centric - after all, Australians cultivate delicious oysters, and why would they follow this rule when their seasons are opposite to those in the northern hemisphere?
The rule almost certainly has its roots in food safety: before the advent of dependable refrigeration, food could spoil quickly in warm weather. Seafood was especially vulnerable, so there was a greater chance of getting food poisoning from it. With fish, you can usually tell by smelling it that it's 'off', but with oysters and bivalves, you can't necessarily sniff out a bad one.
Now, with improved refrigeration and fast shipping, the rule should be changed to 'don't eat oysters harvested from warm waters' - not because of food safety, but because they don't taste as good. Oysters are most delicious when they're harvested from clean, cold water: it improves both the taste and texture. Warm-water oysters are fat and flabby, probably because they spawn during the warmer months. Fortunately, oysters are grown in many parts of the world, so even in months without the 'R' in them, we have access to cold-water oysters from a variety of countries.
However, one rule that does make sense when it comes to oysters is they shouldn't be consumed by those who are pregnant, or who have a compromised immune system. Because they are filter feeders, oysters can contain high levels of bacteria, and cook- ing doesn't always make them safe to eat. Even healthy people should take care when eating oysters: buy them from a dependable source with a high turnover, keep them chilled and use your eyes and nose to check them before putting them in your mouth. If in doubt, throw it out.