Vibrio vulnificus is the bacteria that is more active in coastal waters during warm weather, it’s what’s usually the cause of getting sick from oysters, it’s also the flesh eating bacteria that’ll really mess you up if it gets into an open wound.
Like many other things, ancient people might not have known the reason they got sick from eating oysters in the summer but they could observe it happening and were plenty smart enough to avoid it. The old saying of “if it hurts when you do that then don’t do that” applies. Many things we consider to be wives tales had their root in practical reality like Jewish kosher rules. Ancient Jews might not have known why they got sick when they ate pork but they could see it happening so they decided pork was bad. We know now that pigs raised outdoors are usually full of parasites and you can make it safe by cooking it thoroughly. Internal thermometers weren’t really a thing then so all they knew was when people ate pork they got sick.
If there’s an old rule like the “only eat oysters in months with R” there’s usually a reason behind it. That might or might not still be applicable based upon modern practices but it’s never a good idea to dismiss them out of hand, there’s usually a reason behind them. Ancient people weren’t stupid, they just didn’t have access to the information we do now.
Thanks GC.